Add Balance Sheet report

New first-tab report showing a full breakdown of where money sits:
- Three KPI cards: total assets, total liabilities, net worth
- Proportional stacked bars showing asset and liability composition
- Side-by-side account lists grouped by type (Cash, Savings, ISAs,
  Investments, Pension, Crypto vs Credit Cards, Loans, Mortgages)
- Backend endpoint GET /api/v1/reports/balance-sheet with typed schema
- Balance Sheet is now the default tab on the Reports page

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

backend/app/schemas/report.py

@@ -94,3 +94,27 @@ class SpendingTrendsReport(BaseModel):
     points: list[SpendingTrendPoint]
     categories: list[str]
     currency: str
+
+
+class BalanceSheetAccount(BaseModel):
+    id: str
+    name: str
+    type: str
+    balance: Decimal
+    currency: str
+
+
+class BalanceSheetGroup(BaseModel):
+    label: str
+    type_keys: list[str]
+    accounts: list[BalanceSheetAccount]
+    subtotal: Decimal
+
+
+class BalanceSheetReport(BaseModel):
+    asset_groups: list[BalanceSheetGroup]
+    liability_groups: list[BalanceSheetGroup]
+    total_assets: Decimal
+    total_liabilities: Decimal
+    net_worth: Decimal
+    currency: str