fix terminal theme timing

[Pyasma]

What changed

• Moved shell theme application from useEffect to useLayoutEffect so the document theme is applied before terminal descendants read CSS variables.

Why

• The terminal theme was one render behind because xterm-derived theme values were computed after the DOM theme class/data attribute update.

Impact

• Terminal colors now track the active app theme without the stale-frame mismatch described in issue 3398.

Validation

• cd frontend && npm run typecheck
• cd frontend && npm test -- --run src/renderer/components/XtermTerminal.test.tsx

Fixes #3398