feat: advanced features — dashboard, budgets, history, export

Backend:
- GET /api/v1/dashboard?month=YYYY-MM: KPIs, by_category, 6-month trend, budget alerts
- GET/POST/PUT/DELETE /api/v1/budgets: budget envelopes with spent_cents/remaining_cents
- POST /api/v1/budgets/rollover: copy budgets from M-1 to target month
- GET /api/v1/history?year=YYYY: monthly summary for the year
- GET /api/v1/export/csv|pdf?month=YYYY-MM: StreamingResponse exports (WeasyPrint PDF)
- New schemas: dashboard, budget, history
- Services: dashboard_service, budget_service
- Routers mounted in main.py

Frontend:
- DashboardPage: 4 KPI cards, PieChart (expenses by category), BarChart (6-month trend),
  month navigation, budget alert badges, CSV/PDF export buttons
- BudgetsPage: progress bars (green/orange/red), create/edit form, delete, rollover M-1
- HistoryPage: annual table with month click → dashboard, LineChart revenues/expenses
- CategoriesPage: list by type with create/edit/delete (was missing from Phase 2)
- TransactionsPage: added CSV/PDF export buttons
- App.tsx: full routing with ProtectedRoute + Layout for all authenticated pages
- New hooks: useDashboard, useBudgets (with mutations), useHistory
- API types + client updated for all new endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks/useHistory.ts

@@ -0,0 +1,9 @@
+import { useQuery } from "@tanstack/react-query";
+import { getHistory } from "../api/client";
+
+export function useHistory(year?: number) {
+  return useQuery({
+    queryKey: ["history", year],
+    queryFn: () => getHistory(year),
+  });
+}