docs(02-01): add missing summary for month navigation plan

.planning/phases/02-dashboard-charts-and-layout/02-01-SUMMARY.md

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+---
+phase: 02-dashboard-charts-and-layout
+plan: 01
+subsystem: ui
+tags: [react, react-router, i18n, dashboard, charts]
+
+# Dependency graph
+requires:
+  - phase: 01-design-foundation-and-primitives
+    provides: PageShell, design tokens, shadcn chart primitive
+provides:
+  - useMonthParam hook for URL-based month navigation
+  - MonthNavigator component with prev/next arrows and Select dropdown
+  - Chart and month navigation i18n keys (EN + DE)
+affects: [02-dashboard-charts-and-layout, 03-collapsible-dashboard-sections]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns: [URL-based state via useSearchParams, locale-aware month formatting]
+
+key-files:
+  created:
+    - src/hooks/useMonthParam.ts
+    - src/components/dashboard/MonthNavigator.tsx
+  modified:
+    - src/i18n/en.json
+    - src/i18n/de.json
+
+key-decisions:
+  - "useMonthParam preserves other URL params via setSearchParams callback form"
+  - "navigateMonth uses Date constructor for automatic year rollover"
+  - "MonthNavigator accepts t prop but dropdown uses locale-aware Intl formatting"
+  - "ChartEmptyState already existed from Phase 1 — skipped creation, added i18n keys only"
+
+patterns-established:
+  - "URL-based month state: useMonthParam hook as single source of truth for month selection"
+  - "Month formatting: Date.toLocaleDateString with month:'long', year:'numeric'"
+
+requirements-completed: [UI-DASH-01]
+
+# Metrics
+duration: 2min
+completed: 2026-03-16
+---
+
+# Phase 02, Plan 01: Month Navigation and Chart Infrastructure Summary
+
+**useMonthParam hook and MonthNavigator component for URL-based month selection, plus 10 new chart/navigation i18n keys in EN and DE**
+
+## Performance
+
+- **Duration:** ~2 min
+- **Started:** 2026-03-16T12:02:00Z
+- **Completed:** 2026-03-16T12:03:06Z
+- **Tasks:** 2
+- **Files modified:** 4
+
+## Accomplishments
+- useMonthParam hook reads/writes `?month=YYYY-MM` URL param with current-month fallback and year-rollover navigation
+- MonthNavigator renders prev/next chevron buttons and a Select dropdown of available budget months with locale-aware formatting
+- 10 new i18n keys added for chart labels, month navigation, and empty states in both EN and DE
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Create useMonthParam hook and MonthNavigator component** - `4481950` (feat)
+2. **Task 2: Add chart and month navigation i18n keys** - `42bf1f9` (feat)
+
+## Files Created/Modified
+- `src/hooks/useMonthParam.ts` - URL-based month state hook with navigateMonth for year rollover
+- `src/components/dashboard/MonthNavigator.tsx` - Prev/next arrows + Select dropdown for month selection
+- `src/i18n/en.json` - 10 new dashboard chart and navigation keys
+- `src/i18n/de.json` - Matching German translations
+
+## Decisions Made
+- ChartEmptyState component already existed from Phase 1 — only i18n keys were added, component creation skipped
+- useMonthParam uses setSearchParams callback form to preserve other URL params
+- MonthNavigator uses Date.toLocaleDateString for locale-aware month display
+
+## Deviations from Plan
+
+### Auto-fixed Issues
+
+**1. [Rule 1 - Already Exists] Skipped ChartEmptyState component creation**
+- **Found during:** Task 2 (ChartEmptyState and i18n keys)
+- **Issue:** ChartEmptyState component already existed from Phase 1 setup
+- **Fix:** Skipped creation, added only the i18n keys
+- **Verification:** Component exists and exports correctly
+- **Committed in:** 42bf1f9 (Task 2 commit)
+
+---
+
+**Total deviations:** 1 auto-fixed (1 already exists)
+**Impact on plan:** No scope creep — component existed, only i18n work needed.
+
+## Issues Encountered
+None
+
+## User Setup Required
+None - no external service configuration required.
+
+## Next Phase Readiness
+- Month navigation infrastructure ready for dashboard integration (Plan 03)
+- Chart components (Plan 02) can reference i18n keys
+- All foundational pieces in place for DashboardPage wiring
+
+---
+*Phase: 02-dashboard-charts-and-layout*
+*Completed: 2026-03-16*