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>

README.md

@@ -33,6 +33,7 @@ Runs entirely on your own hardware via Docker Compose. Designed for LAN access w
 - Capital gains reporting (short/long-term by tax year)
 - OHLCV candlestick charts per asset
 - Portfolio visualisations: allocation donut by holding, allocation donut by asset type, cost basis vs current value bar chart, return % per holding bar chart
+- Investment account balances include holding market values in net worth, balance sheet, and accounts list — supports both simple (holdings only) and full cash-flow tracking workflows
 
 ### Reports
 Seven report views: