feat: override trade tab — manual order entry with auto-registration

Backend:
- OverrideOrderRequest/Response Pydantic models with ticker, quantity, price validators
- POST /api/trading/override/order endpoint (enqueue to Redis broker queue)
- auto_register_symbol() module for untracked ticker registration via Symbol Registry
- Unit tests (17) and property-based tests (3 x 100 examples)

Frontend:
- OverrideTradePanel component (order form + positions display)
- Override tab in TradingEngine page with URL search param navigation
- Override Trade button on Trading Controls page
- useSubmitOverrideOrder mutation hook
- MSW handler and 13 component/integration tests

Steering:
- Updated steering docs for Ubuntu dev machine with nvm/Node 24

.kiro/steering/frontend-conventions.md

@@ -19,6 +19,7 @@ fileMatchPattern: "frontend/**"
 
 ## Testing
 - Vitest + MSW (Mock Service Worker) for deterministic tests
+- Requires Node.js 24 via nvm — load before running: `export NVM_DIR="$HOME/.nvm" && [ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh" && nvm use 24`
 - Test setup: `src/test/setup.ts` starts MSW server
 - Mock handlers: `src/test/mocks/handlers.ts`
 - Test helper: `src/test/render.tsx` provides `renderRoute(path)` with QueryClient + Router