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>

backend/app/routers/dashboard.py

@@ -0,0 +1,27 @@
+from fastapi import APIRouter, Depends, Query
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.auth.dependencies import get_current_user
+from app.database import get_session
+from app.models.user import User
+from app.schemas.dashboard import DashboardResponse
+from app.services import dashboard_service
+from app.utils import utcnow
+
+router = APIRouter(prefix="/dashboard", tags=["dashboard"])
+
+
+def _current_month() -> str:
+    now = utcnow()
+    return f"{now.year}-{now.month:02d}"
+
+
+@router.get("", response_model=DashboardResponse)
+async def get_dashboard(
+    month: str = Query(default=None, description="Month YYYY-MM (defaults to current month)"),
+    session: AsyncSession = Depends(get_session),
+    current_user: User = Depends(get_current_user),
+) -> DashboardResponse:
+    if month is None:
+        month = _current_month()
+    return await dashboard_service.get_dashboard(session, current_user.id, month)