docs(04-01): complete locale-aware formatCurrency plan

.planning/REQUIREMENTS.md

@@ -45,7 +45,7 @@ Requirements for this milestone. Each maps to roadmap phases.
 
 ### Bug Fixes
 
-- [ ] **FIX-01**: `formatCurrency` uses the user's locale preference instead of hardcoded `de-DE`
+- [x] **FIX-01**: `formatCurrency` uses the user's locale preference instead of hardcoded `de-DE`
 - [x] **FIX-02**: `InlineEditRow` extracted into a shared component (currently duplicated in BillsTracker, VariableExpenses, DebtTracker)
 
 ## v2 Requirements
@@ -111,7 +111,7 @@ Which phases cover which requirements. Updated during roadmap creation.
 | STATE-02 | Phase 3 | Complete |
 | STATE-03 | Phase 3 | Complete |
 | IXTN-04 | Phase 4 | Pending |
-| FIX-01 | Phase 4 | Pending |
+| FIX-01 | Phase 4 | Complete |
 
 **Coverage:**
 - v1 requirements: 23 total

.planning/ROADMAP.md

@@ -74,7 +74,7 @@ Plans:
   1. All chart fills (donut slices, bar segments) use the semantic category colors from `lib/palette.ts` — "Bills" is the same color in the donut chart as it is in the FinancialOverview table
   2. Chart tooltips display values formatted with the budget's currency (e.g., "1,234.56" not "1234.56")
   3. The `formatCurrency` function uses the user's locale preference from their settings instead of the hardcoded `de-DE` — an English-locale user sees their numbers formatted correctly
-**Plans:** 2 plans
+**Plans:** 1/2 plans executed
 Plans:
 - [ ] 04-01-PLAN.md — TDD: formatCurrency locale parameter fix (FIX-01)
 - [ ] 04-02-PLAN.md — Chart tooltip wiring and locale threading (IXTN-04)
@@ -89,4 +89,4 @@ Phases execute in numeric order: 1 -> 2 -> 3 -> 4
 | 1. Design Token Foundation | 2/2 | Complete   | 2026-03-11 |
 | 2. Layout and Brand Identity | 0/2 | In progress | - |
 | 3. Interaction Quality and Completeness | 0/4 | Not started | - |
-| 4. Chart Polish and Bug Fixes | 0/2 | Not started | - |
+| 4. Chart Polish and Bug Fixes | 1/2 | In Progress|  |

.planning/STATE.md

@@ -3,14 +3,14 @@ gsd_state_version: 1.0
 milestone: v1.0
 milestone_name: milestone
 status: planning
-stopped_at: Completed 03-03-PLAN.md
+stopped_at: Completed 04-01-PLAN.md
-last_updated: "2026-03-11T21:41:42.367Z"
+last_updated: "2026-03-12T08:24:41.212Z"
 last_activity: 2026-03-11 — Roadmap created from requirements and research
 progress:
   total_phases: 4
   completed_phases: 3
-  total_plans: 8
+  total_plans: 10
-  completed_plans: 8
+  completed_plans: 9
   percent: 0
 ---
 
@@ -58,6 +58,7 @@ Progress: [░░░░░░░░░░] 0%
 | Phase 03-interaction-quality-and-completeness P01 | 2m | 2 tasks | 5 files |
 | Phase 03-interaction-quality-and-completeness P02 | 5 | 2 tasks | 3 files |
 | Phase 03-interaction-quality-and-completeness P03 | 5 | 2 tasks | 4 files |
+| Phase 04-chart-polish-and-bug-fixes P01 | 1 | 2 tasks | 2 files |
 
 ## Accumulated Context
 
@@ -87,6 +88,8 @@ Recent decisions affecting current work:
 - [Phase 03-interaction-quality-and-completeness]: CategoriesPage loading state initialized true, set false in finally block to prevent empty-state flash
 - [Phase 03-interaction-quality-and-completeness]: triggerFlash uses two separate state vars (flashRowId/errorRowId) for flash feedback — no race conditions between success and error states
 - [Phase 03-interaction-quality-and-completeness]: Empty tracker sections show tinted skeleton card (not null) — section always visible with palette-tinted placeholders when no items exist
+- [Phase 04-chart-polish-and-bug-fixes]: formatCurrency third parameter defaults to 'en', replacing hardcoded 'de-DE' — all existing 2-arg call sites now produce English formatting (FIX-01)
+- [Phase 04-chart-polish-and-bug-fixes]: Defensive locale || 'en' guard in formatCurrency prevents RangeError when empty string is passed
 
 ### Pending Todos
 
@@ -99,6 +102,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-03-11T21:37:48.889Z
+Last session: 2026-03-12T08:24:41.210Z
-Stopped at: Completed 03-03-PLAN.md
+Stopped at: Completed 04-01-PLAN.md
 Resume file: None

.planning/phases/04-chart-polish-and-bug-fixes/04-01-SUMMARY.md

@@ -0,0 +1,109 @@
+---
+phase: 04-chart-polish-and-bug-fixes
+plan: 01
+subsystem: ui
+tags: [intl, locale, currency, formatting, typescript, vitest]
+
+# Dependency graph
+requires: []
+provides:
+  - "Locale-aware formatCurrency(amount, currency, locale='en') with 'en' default"
+  - "Unit tests covering English, German, USD, zero, negative, and empty-string edge cases"
+affects:
+  - 04-02-chart-tooltips
+  - any component calling formatCurrency
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns:
+    - "Intl.NumberFormat locale parameter passed from call site — no hardcoded locale in utility functions"
+
+key-files:
+  created:
+    - frontend/src/lib/format.test.ts
+  modified:
+    - frontend/src/lib/format.ts
+
+key-decisions:
+  - "formatCurrency third parameter defaults to 'en', replacing hardcoded 'de-DE' — all existing 2-arg call sites now produce English formatting (FIX-01)"
+  - "Defensive locale || 'en' guard prevents RangeError when empty string is passed"
+
+patterns-established:
+  - "TDD pattern: write failing test first (RED commit), then implement (GREEN commit) — verified by vitest run between steps"
+
+requirements-completed: [FIX-01]
+
+# Metrics
+duration: 5min
+completed: 2026-03-12
+---
+
+# Phase 4 Plan 01: Locale-Aware formatCurrency Summary
+
+**`Intl.NumberFormat` locale parameter added to `formatCurrency` with `'en'` default, replacing hardcoded `'de-DE'` — English users now see `1,234.56` instead of `1.234,56`**
+
+## Performance
+
+- **Duration:** 5 min
+- **Started:** 2026-03-12T08:23:13Z
+- **Completed:** 2026-03-12T08:28:00Z
+- **Tasks:** 2 (TDD RED + TDD GREEN)
+- **Files modified:** 2
+
+## Accomplishments
+
+- Added optional `locale` parameter to `formatCurrency` with `'en'` as the default
+- Removed the hardcoded `'de-DE'` locale that was forcing German number formatting for all users
+- Defensive `locale || 'en'` guard prevents `RangeError` on empty string input
+- 8 unit tests cover English formatting, German formatting, USD, zero, negative amounts, and edge cases
+- All 51 frontend tests pass with no regressions
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **TDD RED: Failing tests for locale-aware formatCurrency** - `6ffce76` (test)
+2. **TDD GREEN: Implement locale parameter** - `eb1bb8a` (feat)
+
+_Note: TDD tasks have two commits (test → feat). No refactor needed._
+
+## Files Created/Modified
+
+- `frontend/src/lib/format.test.ts` - 8 unit tests for locale behavior, defaults, and edge cases
+- `frontend/src/lib/format.ts` - Updated function signature with `locale: string = 'en'` parameter
+
+## Decisions Made
+
+- Third parameter defaults to `'en'` (not `'en-US'` or `'de-DE'`) — bare `'en'` is a valid BCP 47 tag and produces comma-grouped, period-decimal output consistent with user expectations for English locale
+- `locale || 'en'` fallback is intentional — empty string is not a valid BCP 47 locale and `Intl.NumberFormat('')` throws `RangeError` in some environments
+
+## Deviations from Plan
+
+None - plan executed exactly as written.
+
+## Issues Encountered
+
+None. Pre-existing `act(...)` warnings in unrelated test files (`InlineEditCell.test.tsx`, `CategoriesPage.test.tsx`) were observed but are out of scope per deviation rules — logged to deferred items.
+
+## User Setup Required
+
+None - no external service configuration required.
+
+## Next Phase Readiness
+
+- `formatCurrency(amount, currency, locale)` is ready for use in chart tooltips (04-02)
+- All call sites currently pass 2 args and will automatically receive English formatting
+- Callers can pass user's `preferred_locale` from API response as the third arg
+
+## Self-Check: PASSED
+
+- `frontend/src/lib/format.ts` — FOUND
+- `frontend/src/lib/format.test.ts` — FOUND
+- `04-01-SUMMARY.md` — FOUND
+- Commit `6ffce76` (test RED) — FOUND
+- Commit `eb1bb8a` (feat GREEN) — FOUND
+
+---
+*Phase: 04-chart-polish-and-bug-fixes*
+*Completed: 2026-03-12*