Include investment holding values in net worth and account balances

- Net worth report, balance sheet, and daily snapshots now add holding
  market values (falling back to cost basis) to investment-type account
  balances (investment, pension, stocks_shares_isa, crypto_wallet)
- Accounts list shows total value for investment accounts with a
  breakdown line ("£X cash + £Y holdings") when both are non-zero
- Add Holding modal gains a "Debit account for this purchase" toggle
  that creates a matching withdrawal transaction, enabling proper cash
  flow tracking for users who fund their brokerage via transfer first
- Both simple (holdings-only) and full cash-flow workflows produce
  correct net worth figures without double-counting

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

backend/app/services/investment_service.py

@@ -142,6 +142,51 @@ async def get_portfolio(db: AsyncSession, user_id: uuid.UUID, base_currency: str
     )
 
 
+async def get_portfolio_value_by_account(
+    db: AsyncSession, user_id: uuid.UUID, base_currency: str
+) -> dict[uuid.UUID, Decimal]:
+    """Return total holding value (in base_currency) keyed by account_id."""
+    result = await db.execute(
+        select(InvestmentHolding).where(
+            InvestmentHolding.user_id == user_id,
+            InvestmentHolding.quantity > 0,
+        )
+    )
+    holdings = result.scalars().all()
+
+    assets: dict[uuid.UUID, Asset] = {}
+    for h in holdings:
+        if h.asset_id not in assets:
+            asset = await _get_asset(db, h.asset_id)
+            if asset:
+                assets[h.asset_id] = asset
+
+    pairs_needed: set[tuple[str, str]] = set()
+    for h in holdings:
+        asset = assets.get(h.asset_id)
+        if not asset:
+            continue
+        if asset.currency != h.currency:
+            pairs_needed.add((asset.currency, h.currency))
+        if h.currency != base_currency:
+            pairs_needed.add((h.currency, base_currency))
+
+    fx_rates: dict[tuple[str, str], Decimal] = {}
+    for from_curr, to_curr in pairs_needed:
+        fx_rates[(from_curr, to_curr)] = await _fetch_fx_rate(db, from_curr, to_curr)
+
+    totals: dict[uuid.UUID, Decimal] = {}
+    for h in holdings:
+        asset = assets.get(h.asset_id)
+        if not asset:
+            continue
+        r = _holding_to_response(h, asset, fx_rates)
+        value = r.current_value if r.current_value is not None else r.cost_basis_total
+        to_base = fx_rates.get((h.currency, base_currency), Decimal("1")) if h.currency != base_currency else Decimal("1")
+        totals[h.account_id] = totals.get(h.account_id, Decimal("0")) + value * to_base
+    return totals
+
+
 async def get_holding(db: AsyncSession, user_id: uuid.UUID, holding_id: uuid.UUID) -> InvestmentHolding | None:
     result = await db.execute(
         select(InvestmentHolding).where(