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)

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

backend/app/services/report_service.py

@@ -11,7 +11,11 @@ from app.db.models.budget import Budget
 from app.db.models.category import Category
 from app.db.models.net_worth_snapshot import NetWorthSnapshot
 from app.db.models.transaction import Transaction
+from app.core.security import decrypt_field
 from app.schemas.report import (
+    BalanceSheetAccount,
+    BalanceSheetGroup,
+    BalanceSheetReport,
     BudgetVsActualItem,
     BudgetVsActualReport,
     CashFlowPoint,
@@ -329,6 +333,75 @@ async def get_spending_trends(
     return SpendingTrendsReport(points=points, categories=categories, currency="GBP")
 
 
+async def get_balance_sheet(
+    db: AsyncSession, user_id: uuid.UUID, base_currency: str
+) -> BalanceSheetReport:
+    result = await db.execute(
+        select(Account).where(
+            Account.user_id == user_id,
+            Account.is_active == True,  # noqa: E712
+            Account.deleted_at.is_(None),
+        )
+    )
+    accounts = result.scalars().all()
+
+    ASSET_GROUPS = [
+        ("Cash & Current Accounts", ["checking", "cash"]),
+        ("Savings", ["savings"]),
+        ("ISAs", ["cash_isa", "stocks_shares_isa"]),
+        ("Investments & Pension", ["investment", "pension"]),
+        ("Crypto", ["crypto_wallet"]),
+        ("Other Assets", ["other"]),
+    ]
+    LIABILITY_GROUPS = [
+        ("Credit Cards", ["credit_card"]),
+        ("Loans", ["loan"]),
+        ("Mortgages", ["mortgage"]),
+    ]
+
+    def build_groups(group_defs: list) -> list[BalanceSheetGroup]:
+        groups = []
+        covered: set[str] = set()
+        for label, type_keys in group_defs:
+            covered.update(type_keys)
+            members = [a for a in accounts if a.type in type_keys]
+            if not members:
+                continue
+            acct_items = []
+            for a in members:
+                name = decrypt_field(bytes(a.name_enc)) if a.name_enc else "Account"
+                bal = abs(a.current_balance or Decimal("0"))
+                acct_items.append(BalanceSheetAccount(
+                    id=str(a.id),
+                    name=name,
+                    type=a.type,
+                    balance=bal,
+                    currency=a.currency or base_currency,
+                ))
+            groups.append(BalanceSheetGroup(
+                label=label,
+                type_keys=type_keys,
+                accounts=acct_items,
+                subtotal=sum(i.balance for i in acct_items),
+            ))
+        return groups
+
+    asset_groups = build_groups(ASSET_GROUPS)
+    liability_groups = build_groups(LIABILITY_GROUPS)
+
+    total_assets = sum(g.subtotal for g in asset_groups)
+    total_liabilities = sum(g.subtotal for g in liability_groups)
+
+    return BalanceSheetReport(
+        asset_groups=asset_groups,
+        liability_groups=liability_groups,
+        total_assets=total_assets,
+        total_liabilities=total_liabilities,
+        net_worth=total_assets - total_liabilities,
+        currency=base_currency,
+    )
+
+
 async def take_net_worth_snapshot(db: AsyncSession, user_id: uuid.UUID, base_currency: str) -> None:
     today = date.today()
     existing = await db.execute(