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/api/v1/reports.py

@@ -6,6 +6,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.dependencies import get_current_user, get_db
 from app.db.models.user import User
 from app.schemas.report import (
+    BalanceSheetReport,
     BudgetVsActualReport,
     CashFlowReport,
     CategoryBreakdownReport,
@@ -80,3 +81,11 @@ async def spending_trends(
     current_user: User = Depends(get_current_user),
 ):
     return await report_service.get_spending_trends(db, current_user.id, months)
+
+
+@router.get("/balance-sheet", response_model=BalanceSheetReport)
+async def balance_sheet(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    return await report_service.get_balance_sheet(db, current_user.id, current_user.base_currency)