feat: implement inventory CRUD UI components (#18 #19 #20 #21)

Week 2 core inventory management:

**Composables:**
- useInventory: Full CRUD operations for inventory items
- useUnits: Unit fetching and conversion helpers
- useTags: Tag fetching and category filtering

**Components:**
- InventoryList (#18): Grid view with loading/empty/error states
- InventoryCard: Item card with image, quantity controls, tags, expiry
- AddItemForm (#19): Form with tag picker, unit selector, validation
- EditItemModal (#20): Modal form for editing existing items
- Delete functionality (#21): Confirm dialog + cascade tag cleanup

**Features:**
- Quantity quick-actions (+/- buttons on cards)
- Auto-delete when quantity reaches zero
- Expiry date tracking with color-coded warnings
- Tag selection by category in add form
- Responsive grid layout (1-4 columns)
- Product image display from barcode cache
- Form validation and loading states

Closes #18, #19, #20, #21

app/composables/useUnits.ts

@@ -0,0 +1,53 @@
+export const useUnits = () => {
+  const supabase = useSupabase()
+
+  /**
+   * Get all units
+   */
+  const getUnits = async () => {
+    const { data, error } = await supabase
+      .from('units')
+      .select('*')
+      .order('unit_type', { ascending: true })
+      .order('name', { ascending: true })
+
+    if (error) {
+      console.error('Error fetching units:', error)
+      return { data: null, error }
+    }
+
+    return { data, error: null }
+  }
+
+  /**
+   * Get default unit for a type
+   */
+  const getDefaultUnit = async (unitType: 'weight' | 'volume' | 'count' | 'custom') => {
+    const { data, error } = await supabase
+      .from('units')
+      .select('*')
+      .eq('unit_type', unitType)
+      .eq('is_default', true)
+      .single()
+
+    if (error) {
+      console.error('Error fetching default unit:', error)
+      return { data: null, error }
+    }
+
+    return { data, error: null }
+  }
+
+  /**
+   * Convert quantity between units
+   */
+  const convertUnit = (quantity: number, fromFactor: number, toFactor: number): number => {
+    return (quantity * fromFactor) / toFactor
+  }
+
+  return {
+    getUnits,
+    getDefaultUnit,
+    convertUnit
+  }
+}