feat: add comprehensive E2E testing guide (#40)

- Complete manual testing guide (E2E_TESTING.md)
- 30+ test scenarios across 10 categories
- Critical user flows documented
- Test data seed script
- Bug report template
- Cross-browser test matrix
- Accessibility testing
- Performance testing
- Error handling tests

Test categories:
1. Authentication (4 tests)
2. Inventory Management (5 tests)
3. Barcode Scanning (3 tests)
4. Tag Management (3 tests)
5. PWA Installation (3 tests)
6. Offline Functionality (3 tests)
7. Responsive Design (3 tests)
8. Performance (2 tests)
9. Accessibility (2 tests)
10. Error Handling (2 tests)

Ready for pre-release testing.

Closes #40

docs/E2E_TESTING.md

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+# End-to-End Testing Guide
+
+## Overview
+
+This guide covers manual end-to-end testing of the Pantry MVP. These tests ensure all critical user flows work correctly before release.
+
+## Test Environment
+
+**Requirements:**
+- Browser: Chrome/Edge, Firefox, or Safari
+- Resolution: 1920x1080 (desktop) and 390x844 (mobile)
+- Network: Both online and offline modes
+- Data: Fresh database with seed data
+
+## Critical User Flows
+
+### 1. Authentication Flow
+
+#### Test 1.1: Sign Up
+**Priority:** HIGH
+
+1. Navigate to app home page
+2. Click "Sign Up" or similar
+3. Enter email: test@example.com
+4. Enter password: SecurePass123!
+5. Confirm password
+6. Submit form
+
+**Expected Results:**
+- ✅ Validation errors if password too weak
+- ✅ Account created successfully
+- ✅ Redirected to home/dashboard
+- ✅ Welcome message shown (optional)
+
+#### Test 1.2: Sign In
+**Priority:** HIGH
+
+1. Sign out if signed in
+2. Navigate to home page
+3. Click "Sign In"
+4. Enter correct credentials
+5. Submit form
+
+**Expected Results:**
+- ✅ Successfully signed in
+- ✅ Redirected to dashboard
+- ✅ User menu shows email/name
+
+#### Test 1.3: Sign In (Wrong Credentials)
+**Priority:** MEDIUM
+
+1. Try signing in with wrong password
+2. Try signing in with non-existent email
+
+**Expected Results:**
+- ✅ Error message shown
+- ✅ Form not cleared (email retained)
+- ✅ No redirect
+- ✅ Try again allowed
+
+#### Test 1.4: Sign Out
+**Priority:** HIGH
+
+1. While signed in, click user menu
+2. Click "Sign Out"
+
+**Expected Results:**
+- ✅ Signed out successfully
+- ✅ Redirected to sign-in page
+- ✅ Cannot access protected pages
+
+### 2. Inventory Management
+
+#### Test 2.1: View Inventory List
+**Priority:** HIGH
+
+1. Sign in
+2. Navigate to home/inventory page
+
+**Expected Results:**
+- ✅ List of pantry items displayed
+- ✅ Items show: name, quantity, unit, location tags
+- ✅ Empty state if no items
+- ✅ Loading state while fetching
+
+#### Test 2.2: Add Item Manually
+**Priority:** HIGH
+
+1. Navigate to inventory
+2. Click "Add Item" button
+3. Enter item details:
+   - Name: "Olive Oil"
+   - Quantity: 1
+   - Unit: L (liter)
+   - Tags: pantry, cooking-oil
+4. Submit
+
+**Expected Results:**
+- ✅ Form validation works
+- ✅ Item appears in list immediately
+- ✅ Success message shown
+- ✅ Form cleared for next entry
+
+#### Test 2.3: Edit Item
+**Priority:** HIGH
+
+1. Click on an existing item
+2. Click "Edit" button
+3. Change quantity from 1 to 0.5
+4. Save changes
+
+**Expected Results:**
+- ✅ Modal/form opens with current values
+- ✅ Changes saved successfully
+- ✅ List updated immediately
+- ✅ No page reload needed
+
+#### Test 2.4: Delete Item
+**Priority:** HIGH
+
+1. Click on an item
+2. Click "Delete" button
+3. Confirm deletion
+
+**Expected Results:**
+- ✅ Confirmation dialog shown
+- ✅ Item removed from list
+- ✅ Success message shown
+- ✅ Can undo (optional)
+
+#### Test 2.5: Filter by Tag
+**Priority:** MEDIUM
+
+1. View inventory list
+2. Click on a tag filter (e.g., "fridge")
+
+**Expected Results:**
+- ✅ Only items with that tag shown
+- ✅ Filter state persists
+- ✅ Clear filter button available
+- ✅ Item count updated
+
+### 3. Barcode Scanning
+
+#### Test 3.1: Scan Known Product
+**Priority:** HIGH
+
+1. Navigate to Scan page
+2. Allow camera permissions
+3. Scan a barcode (e.g., Coca-Cola)
+
+**Expected Results:**
+- ✅ Camera opens
+- ✅ Barcode detected
+- ✅ Product info fetched from Open Food Facts
+- ✅ Pre-filled add form shown
+- ✅ Can edit before adding
+
+#### Test 3.2: Scan Unknown Product
+**Priority:** MEDIUM
+
+1. Scan a barcode not in database
+2. Manual entry form shown
+
+**Expected Results:**
+- ✅ "Product not found" message
+- ✅ Option to add manually
+- ✅ Barcode pre-filled
+- ✅ Can still save item
+
+#### Test 3.3: Camera Permission Denied
+**Priority:** MEDIUM
+
+1. Block camera permission
+2. Try to scan
+
+**Expected Results:**
+- ✅ Permission prompt shown
+- ✅ Helpful error message
+- ✅ Instructions to enable camera
+- ✅ Option to add manually
+
+### 4. Tag Management
+
+#### Test 4.1: Create Tag
+**Priority:** MEDIUM
+
+1. Go to Settings → Tags
+2. Click "Add Tag"
+3. Enter name: "freezer"
+4. Pick color: blue
+5. Save
+
+**Expected Results:**
+- ✅ Tag created successfully
+- ✅ Appears in tag list
+- ✅ Available in item forms
+- ✅ Color applied correctly
+
+#### Test 4.2: Edit Tag
+**Priority:** LOW
+
+1. Click on existing tag
+2. Change name or color
+3. Save
+
+**Expected Results:**
+- ✅ Tag updated
+- ✅ All items with tag reflect changes
+- ✅ No broken references
+
+#### Test 4.3: Delete Tag
+**Priority:** LOW
+
+1. Click delete on a tag
+2. Confirm
+
+**Expected Results:**
+- ✅ Confirmation shown if tag in use
+- ✅ Tag removed
+- ✅ Items keep other tags
+- ✅ No app crashes
+
+### 5. PWA Installation
+
+#### Test 5.1: Install Prompt (Desktop)
+**Priority:** MEDIUM
+
+1. Visit app on Chrome (desktop)
+2. Wait 3 seconds
+
+**Expected Results:**
+- ✅ Install banner appears
+- ✅ Shows app icon and name
+- ✅ Install button works
+- ✅ Can dismiss and won't show for 7 days
+
+#### Test 5.2: Install from Settings
+**Priority:** MEDIUM
+
+1. Go to Settings → App
+2. Click "Install App" button
+
+**Expected Results:**
+- ✅ Browser install dialog opens
+- ✅ App installs to desktop/home screen
+- ✅ Launches in standalone mode
+- ✅ Icon and name correct
+
+#### Test 5.3: iOS Safari Add to Home Screen
+**Priority:** HIGH (iOS users)
+
+1. Open app in Safari (iOS)
+2. Tap Share button
+3. Tap "Add to Home Screen"
+4. Confirm
+
+**Expected Results:**
+- ✅ Icon added to home screen
+- ✅ Opens in standalone mode
+- ✅ Splash screen shown
+- ✅ No Safari UI
+
+### 6. Offline Functionality
+
+#### Test 6.1: Work Offline
+**Priority:** HIGH
+
+1. Load app while online
+2. Navigate to all pages
+3. Disconnect internet
+4. Try navigating again
+
+**Expected Results:**
+- ✅ Offline banner appears
+- ✅ Previously visited pages load
+- ✅ Cached data visible
+- ✅ No white screens or errors
+
+#### Test 6.2: Offline Indicator
+**Priority:** MEDIUM
+
+1. Go offline
+2. Check for visual feedback
+
+**Expected Results:**
+- ✅ "You're offline" banner at top
+- ✅ Banner is amber/warning color
+- ✅ Icon indicates offline status
+
+#### Test 6.3: Return Online
+**Priority:** MEDIUM
+
+1. Go offline
+2. Wait a few seconds
+3. Go online
+
+**Expected Results:**
+- ✅ "Back online!" banner (green)
+- ✅ Auto-hides after 3 seconds
+- ✅ Data syncs (if pending changes)
+
+### 7. Responsive Design
+
+#### Test 7.1: Mobile View
+**Priority:** HIGH
+
+1. Resize browser to 390px wide
+2. Navigate all pages
+
+**Expected Results:**
+- ✅ Navigation adapts (hamburger menu)
+- ✅ Lists stack vertically
+- ✅ Buttons are touch-friendly
+- ✅ No horizontal scroll
+- ✅ Text readable without zoom
+
+#### Test 7.2: Tablet View
+**Priority:** MEDIUM
+
+1. Resize to 768px wide
+
+**Expected Results:**
+- ✅ Layout adapts
+- ✅ 2-column grids where appropriate
+- ✅ Navigation hybrid or drawer
+
+#### Test 7.3: Desktop View
+**Priority:** MEDIUM
+
+1. View at 1920px wide
+
+**Expected Results:**
+- ✅ Full navigation visible
+- ✅ Multi-column layouts
+- ✅ Content not stretched too wide
+- ✅ Whitespace used effectively
+
+### 8. Performance
+
+#### Test 8.1: Page Load Time
+**Priority:** HIGH
+
+1. Clear cache
+2. Load home page
+3. Measure time to interactive
+
+**Expected Results:**
+- ✅ < 3 seconds on 4G
+- ✅ < 1 second on repeat visit
+- ✅ Loading indicators shown
+
+#### Test 8.2: Lighthouse Score
+**Priority:** MEDIUM
+
+1. Open DevTools → Lighthouse
+2. Run "Progressive Web App" audit
+
+**Expected Results:**
+- ✅ PWA score: 90+
+- ✅ Performance: 80+
+- ✅ Accessibility: 90+
+- ✅ Best Practices: 90+
+
+### 9. Accessibility
+
+#### Test 9.1: Keyboard Navigation
+**Priority:** HIGH
+
+1. Use only keyboard (Tab, Enter, Esc)
+2. Navigate entire app
+
+**Expected Results:**
+- ✅ All interactive elements reachable
+- ✅ Focus indicators visible
+- ✅ Modals can be closed with Esc
+- ✅ Logical tab order
+
+#### Test 9.2: Screen Reader
+**Priority:** MEDIUM
+
+1. Enable VoiceOver (Mac) or NVDA (Windows)
+2. Navigate app
+
+**Expected Results:**
+- ✅ All text announced
+- ✅ Images have alt text
+- ✅ Form labels read correctly
+- ✅ Buttons have meaningful labels
+
+### 10. Error Handling
+
+#### Test 10.1: Network Error
+**Priority:** HIGH
+
+1. Start action (add item)
+2. Disable network mid-request
+3. Check behavior
+
+**Expected Results:**
+- ✅ Error message shown
+- ✅ Action can be retried
+- ✅ Data not lost
+- ✅ No crash
+
+#### Test 10.2: Server Error (500)
+**Priority:** MEDIUM
+
+1. Trigger server error (if possible)
+
+**Expected Results:**
+- ✅ User-friendly error message
+- ✅ No stack traces visible
+- ✅ Option to try again
+- ✅ Logs sent to monitoring (optional)
+
+## Test Data Setup
+
+### Seed Data Script
+
+```sql
+-- Run in Supabase SQL Editor
+-- Insert test household
+INSERT INTO households (id, name) VALUES 
+  ('test-household-1', 'Test Household');
+
+-- Insert test tags
+INSERT INTO tags (id, household_id, name, color) VALUES
+  ('tag-1', 'test-household-1', 'fridge', '#3b82f6'),
+  ('tag-2', 'test-household-1', 'pantry', '#10b981'),
+  ('tag-3', 'test-household-1', 'freezer', '#6366f1');
+
+-- Insert test units
+INSERT INTO units (id, household_id, name, abbreviation, base_unit, conversion_factor) VALUES
+  ('unit-1', 'test-household-1', 'liter', 'L', 'L', 1),
+  ('unit-2', 'test-household-1', 'milliliter', 'mL', 'L', 0.001),
+  ('unit-3', 'test-household-1', 'kilogram', 'kg', 'kg', 1);
+
+-- Insert test items
+INSERT INTO pantry_items (household_id, name, quantity, unit_id) VALUES
+  ('test-household-1', 'Milk', 1, 'unit-1'),
+  ('test-household-1', 'Rice', 2, 'unit-3'),
+  ('test-household-1', 'Olive Oil', 0.5, 'unit-1');
+```
+
+## Test Execution Checklist
+
+**Before Testing:**
+- [ ] Fresh database with seed data
+- [ ] Clear browser cache
+- [ ] Clear localStorage
+- [ ] Unregister service workers
+- [ ] Sign out all accounts
+
+**Test Environments:**
+- [ ] Chrome (Windows/Mac/Linux)
+- [ ] Firefox (Windows/Mac/Linux)
+- [ ] Safari (Mac/iOS)
+- [ ] Mobile Chrome (Android)
+- [ ] Mobile Safari (iOS)
+
+**Test Scenarios:**
+- [ ] Authentication (4 tests)
+- [ ] Inventory Management (5 tests)
+- [ ] Barcode Scanning (3 tests)
+- [ ] Tag Management (3 tests)
+- [ ] PWA Installation (3 tests)
+- [ ] Offline Functionality (3 tests)
+- [ ] Responsive Design (3 tests)
+- [ ] Performance (2 tests)
+- [ ] Accessibility (2 tests)
+- [ ] Error Handling (2 tests)
+
+## Bug Report Template
+
+```
+**Title:** Short description
+
+**Priority:** High / Medium / Low
+
+**Environment:**
+- Browser: Chrome 120.0
+- OS: Windows 11
+- Device: Desktop 1920x1080
+
+**Steps to Reproduce:**
+1. Navigate to...
+2. Click on...
+3. Enter...
+
+**Expected Result:**
+What should happen
+
+**Actual Result:**
+What actually happened
+
+**Screenshots/Video:**
+[Attach if applicable]
+
+**Console Errors:**
+[Copy any errors from DevTools console]
+
+**Workaround:**
+[If known]
+```
+
+## Sign-off
+
+**Tested by:** [Name]  
+**Date:** [Date]  
+**Environment:** [Browser, OS]  
+**Total Tests:** [X]  
+**Passed:** [X]  
+**Failed:** [X]  
+**Blocked:** [X]
+
+**Critical Issues Found:** [List or "None"]  
+**Recommendation:** ✅ Ready for Release / ❌ Needs Fixes
+
+---
+
+**Next Steps:**
+- Fix critical issues
+- Retest failed scenarios
+- Document known limitations
+- Prepare release notes