docs(34-03): complete locale-aware formatter integration plan summary

- All 5 tasks verified complete: useLanguage hook, formatPrice/formatWeight
  with Intl.NumberFormat, useFormatters locale wiring, formatter tests
- 15 tests passing, build clean, CURRENCY_SYMBOLS removed

.planning/phases/34-i18n-foundation/34-03-SUMMARY.md

@@ -0,0 +1,130 @@
+---
+phase: 34-i18n-foundation
+plan: "03"
+subsystem: ui
+tags: [i18n, formatters, intl, locale, react-hooks, typescript]
+
+# Dependency graph
+requires:
+  - phase: 34-i18n-foundation/34-01
+    provides: i18n infrastructure, translation framework, useSetting hook patterns
+provides:
+  - Locale-aware formatPrice using Intl.NumberFormat (en: "$1,234.56", de: "1.234,56 €")
+  - Locale-aware formatWeight using Intl.NumberFormat (en: "1,234g", de: "1.234g")
+  - useLanguage hook reading language from settings with "en" fallback
+  - useFormatters hook wiring locale into all format calls
+  - Formatter test suite covering null, en locale, de locale, unit conversions
+affects: [34-04, 34-05, 34-06, 34-07, 34-08]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns:
+    - "Intl.NumberFormat for all number/currency formatting instead of manual symbol lookup"
+    - "locale parameter added as third argument (defaulting to 'en') for backward compatibility"
+    - "useLanguage follows same pattern as useWeightUnit/useCurrency: useSetting + VALID_* array + default"
+
+key-files:
+  created:
+    - src/client/hooks/useLanguage.ts
+    - tests/formatters.test.ts
+  modified:
+    - src/client/lib/formatters.ts
+    - src/client/hooks/useFormatters.ts
+
+key-decisions:
+  - "locale parameter defaults to 'en' so existing callers without locale continue to work"
+  - "CURRENCY_SYMBOLS constant removed — Intl.NumberFormat handles symbols natively"
+  - "VALID_LANGUAGES ['en', 'de'] validates DB value before returning; invalid falls back to 'en' (T-34-04 threat mitigation)"
+
+patterns-established:
+  - "Locale-aware formatting: all number/price/weight formatters accept locale as third arg"
+  - "Settings hook pattern: useSetting + VALID_* array const + default fallback"
+
+requirements-completed: [D-04, D-09, D-10]
+
+# Metrics
+duration: 15min
+completed: 2026-04-18
+---
+
+# Phase 34 Plan 03: Locale-Aware Formatter Integration Summary
+
+**Intl.NumberFormat-based locale-aware formatPrice and formatWeight with useLanguage hook — German locale shows "1.234,56 €", English shows "$1,234.56"**
+
+## Performance
+
+- **Duration:** ~15 min
+- **Started:** 2026-04-18T12:15:00Z
+- **Completed:** 2026-04-18T12:30:00Z
+- **Tasks:** 5
+- **Files modified:** 4
+
+## Accomplishments
+
+- `useLanguage()` hook reads language from settings DB, validates against `VALID_LANGUAGES`, falls back to "en"
+- `formatPrice()` updated to use `Intl.NumberFormat(locale, { style: "currency", currency })` — CURRENCY_SYMBOLS removed
+- `formatWeight()` updated to use `Intl.NumberFormat(locale, { minimumFractionDigits, maximumFractionDigits })` for locale-aware separators
+- `useFormatters()` extended to call `useLanguage()` and pass locale to both formatters, exposing locale in return value
+- 15-test suite covering null, en/de locales, unit conversions, JPY special case, large number thousands separators
+
+## Task Commits
+
+All tasks were implemented in a prior commit and verified as complete at plan execution time:
+
+- **Tasks 1-4: locale-aware formatters and useLanguage hook** - `f759dd0` (feat)
+- **Task 5: formatter tests** - `f759dd0` (feat/test)
+
+Note: Implementation pre-existed this plan's execution in commit `f759dd0` (feat(i18n): locale-aware formatters and useLanguage hook). All acceptance criteria verified as passing.
+
+## Files Created/Modified
+
+- `src/client/hooks/useLanguage.ts` — Hook reading "language" setting, returning Language type with "en" fallback, exports VALID_LANGUAGES
+- `src/client/lib/formatters.ts` — formatPrice and formatWeight updated with locale parameter and Intl.NumberFormat; CURRENCY_SYMBOLS removed
+- `src/client/hooks/useFormatters.ts` — Extended with useLanguage import, locale passed to both formatters, locale in return object
+- `tests/formatters.test.ts` — 15 tests for formatPrice and formatWeight across locales, units, null, and edge cases
+
+## Decisions Made
+
+- Locale parameter defaults to "en" to preserve backward compatibility with callers that don't pass locale
+- CURRENCY_SYMBOLS constant removed entirely — Intl.NumberFormat handles currency symbols natively for all currencies
+- T-34-04 threat mitigation applied: VALID_LANGUAGES validation ensures untrusted DB values can't cause unexpected locale behavior
+
+## Deviations from Plan
+
+None - plan executed exactly as written. All files were already implemented in the correct state matching the plan's acceptance criteria.
+
+## Issues Encountered
+
+None.
+
+## Known Stubs
+
+None — all formatters produce real locale-aware output from Intl.NumberFormat.
+
+## Threat Flags
+
+None — no new network endpoints, auth paths, or trust boundaries introduced. The T-34-04 threat (tampering via settings DB language value) is mitigated by VALID_LANGUAGES validation in useLanguage.
+
+## User Setup Required
+
+None - no external service configuration required.
+
+## Next Phase Readiness
+
+- Locale-aware formatters are ready for phase 34-04 (UI locale switching) and 34-05 (currency display)
+- useLanguage hook is consumed by useFormatters, which is used app-wide via the `useFormatters()` hook pattern
+- Build passes cleanly, all 15 formatter tests pass
+
+## Self-Check: PASSED
+
+- `src/client/hooks/useLanguage.ts` — FOUND
+- `src/client/lib/formatters.ts` — FOUND (Intl.NumberFormat: 2 occurrences, CURRENCY_SYMBOLS: 0 occurrences)
+- `src/client/hooks/useFormatters.ts` — FOUND (useLanguage + locale: 6 occurrences)
+- `tests/formatters.test.ts` — FOUND (15 tests, all pass)
+- Build: PASSED (built in 937ms)
+- Commit f759dd0 — FOUND
+
+---
+*Phase: 34-i18n-foundation*
+*Completed: 2026-04-18*