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/schemas/budget.py

@@ -0,0 +1,51 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, field_validator
+
+
+class BudgetCreate(BaseModel):
+    category_id: uuid.UUID
+    month: str  # YYYY-MM
+    limit_cents: int
+
+    @field_validator("limit_cents")
+    @classmethod
+    def limit_positive(cls, v: int) -> int:
+        if v <= 0:
+            raise ValueError("limit_cents must be positive")
+        return v
+
+    @field_validator("month")
+    @classmethod
+    def month_format(cls, v: str) -> str:
+        parts = v.split("-")
+        if len(parts) != 2 or not all(p.isdigit() for p in parts):
+            raise ValueError("month must be in YYYY-MM format")
+        return v
+
+
+class BudgetUpdate(BaseModel):
+    limit_cents: int
+
+    @field_validator("limit_cents")
+    @classmethod
+    def limit_positive(cls, v: int) -> int:
+        if v <= 0:
+            raise ValueError("limit_cents must be positive")
+        return v
+
+
+class BudgetResponse(BaseModel):
+    id: uuid.UUID
+    category_id: uuid.UUID
+    category_name: str
+    category_color: str | None
+    month: str
+    limit_cents: int
+    spent_cents: int
+    remaining_cents: int
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}