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>

frontend/src/api/reports.ts

@@ -121,3 +121,32 @@ export async function getSpendingTrends(months = 6): Promise<SpendingTrendsRepor
   const r = await api.get("/api/v1/reports/spending-trends", { params: { months } });
   return r.data;
 }
+
+export interface BalanceSheetAccount {
+  id: string;
+  name: string;
+  type: string;
+  balance: number;
+  currency: string;
+}
+
+export interface BalanceSheetGroup {
+  label: string;
+  type_keys: string[];
+  accounts: BalanceSheetAccount[];
+  subtotal: number;
+}
+
+export interface BalanceSheetReport {
+  asset_groups: BalanceSheetGroup[];
+  liability_groups: BalanceSheetGroup[];
+  total_assets: number;
+  total_liabilities: number;
+  net_worth: number;
+  currency: string;
+}
+
+export async function getBalanceSheet(): Promise<BalanceSheetReport> {
+  const r = await api.get("/api/v1/reports/balance-sheet");
+  return r.data;
+}