docs(04): create phase plan

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

@@ -0,0 +1,103 @@
+---
+phase: 04-chart-polish-and-bug-fixes
+plan: 01
+type: tdd
+wave: 1
+depends_on: []
+files_modified:
+  - frontend/src/lib/format.ts
+  - frontend/src/lib/format.test.ts
+autonomous: true
+requirements:
+  - FIX-01
+
+must_haves:
+  truths:
+    - "formatCurrency no longer hardcodes 'de-DE' — the default locale is 'en'"
+    - "formatCurrency accepts an optional third locale parameter"
+    - "Calling formatCurrency(1234.56, 'EUR', 'de') produces German-formatted output"
+    - "Calling formatCurrency(1234.56, 'USD', 'en') produces English-formatted output"
+    - "Calling formatCurrency(1234.56, 'EUR') without locale uses 'en' default, not 'de-DE'"
+  artifacts:
+    - path: "frontend/src/lib/format.ts"
+      provides: "Locale-aware formatCurrency function"
+      contains: "locale"
+    - path: "frontend/src/lib/format.test.ts"
+      provides: "Unit tests for formatCurrency locale behavior"
+      min_lines: 20
+  key_links:
+    - from: "frontend/src/lib/format.ts"
+      to: "Intl.NumberFormat"
+      via: "locale parameter passed as first arg"
+      pattern: "Intl\\.NumberFormat\\(locale"
+---
+
+<objective>
+Fix the hardcoded `'de-DE'` locale in `formatCurrency` by adding an optional `locale` parameter with `'en'` as the default. This is the foundation for IXTN-04 (chart tooltips) and ensures all currency formatting respects the user's locale preference.
+
+Purpose: FIX-01 — English-locale users currently see German number formatting everywhere
+Output: Updated `format.ts` with locale parameter, comprehensive unit tests
+</objective>
+
+<execution_context>
+@/home/jean-luc-makiola/.claude/get-shit-done/workflows/execute-plan.md
+@/home/jean-luc-makiola/.claude/get-shit-done/templates/summary.md
+</execution_context>
+
+<context>
+@.planning/PROJECT.md
+@.planning/ROADMAP.md
+@.planning/phases/04-chart-polish-and-bug-fixes/04-RESEARCH.md
+@frontend/src/lib/format.ts
+</context>
+
+<feature>
+  <name>Locale-aware formatCurrency</name>
+  <files>frontend/src/lib/format.ts, frontend/src/lib/format.test.ts</files>
+  <behavior>
+    - formatCurrency(1234.56, 'EUR', 'en') returns a string containing "1,234.56" (English grouping)
+    - formatCurrency(1234.56, 'EUR', 'de') returns a string containing "1.234,56" (German grouping)
+    - formatCurrency(1234.56, 'USD', 'en') returns a string containing "$" and "1,234.56"
+    - formatCurrency(1234.56, 'EUR') with NO locale arg uses 'en' default — does NOT produce German formatting
+    - formatCurrency(0, 'EUR', 'en') returns a string containing "0.00"
+    - formatCurrency(-500, 'EUR', 'en') returns a string containing "-" and "500.00"
+    - formatCurrency(1234.56, 'EUR', '') falls back gracefully (does not throw RangeError)
+  </behavior>
+  <implementation>
+    Update `frontend/src/lib/format.ts`:
+
+    ```ts
+    export function formatCurrency(
+      amount: number,
+      currency: string = 'EUR',
+      locale: string = 'en'
+    ): string {
+      return new Intl.NumberFormat(locale || 'en', {
+        style: 'currency',
+        currency,
+      }).format(amount)
+    }
+    ```
+
+    Key points:
+    - Third parameter `locale` defaults to `'en'` (replacing hardcoded `'de-DE'`)
+    - Defensive `locale || 'en'` guards against empty string (which throws RangeError)
+    - All existing call sites pass only 2 args — they will now get English formatting instead of German
+    - This is intentional and correct per FIX-01
+  </implementation>
+</feature>
+
+<verification>
+cd frontend && bun vitest run src/lib/format.test.ts
+</verification>
+
+<success_criteria>
+- format.test.ts has at least 6 test cases covering locale variations, defaults, and edge cases
+- All tests pass green
+- formatCurrency signature has 3 parameters: amount, currency, locale
+- No hardcoded 'de-DE' remains in format.ts
+</success_criteria>
+
+<output>
+After completion, create `.planning/phases/04-chart-polish-and-bug-fixes/04-01-SUMMARY.md`
+</output>