Story: PnL tracking improvements

Improve OreStudio's PnL tracking to match the standard set by leading retail platforms. Drawing on the Lightyear PnL tracking approach, this story introduces: FX rate locking at cash-event time (deposits and withdrawals fix their exchange rate permanently), timeframe-scoped PnL so charts reflect the selected window, a clean split between total-portfolio-value and investment-only PnL views, two-phase sell-order calculations (intraday estimate → EOD final), and user-selectable cost basis methods (weighted average or FIFO).

• FX rates are locked at the moment each deposit or withdrawal is recorded; subsequent FX moves do not alter historical cost basis.
• PnL figures respect the selected chart timeframe (1D, 1M, YTD, all-time).
• The UI offers two chart modes: total portfolio value (including uninvested cash) and investment-only PnL (excluding cash movements).
• Sell-order P&L shows an intraday estimate immediately and updates to EOD-rate final figures once settlement closes.
• Accounts can be configured to use either weighted average cost or FIFO cost basis; the method is reflected consistently in P&L display and in Capital Gains Statements.

• Approach informed by Lightyear PnL tracking approach (external knowledge doc with full source attribution).

• Tax reporting integration (tracked separately).
• Performance attribution beyond the simple money-weighted formula.