fix(ui): isolate theme hook from refreshable provider

[Dragonzz27]

Fixes #2872.

Summary

• Move the theme context and useThemeMode into a non-component module, leaving themeMode.tsx with only its provider export.
• Update consumers while preserving the public app-extension re-export.
• Cover fallback behavior, system preference, explicit/persisted choice, and single-palette extensions.

Verification

From ui/:

• yarn exec eslint src/theme --max-warnings 0 — passes; the original module fails this check with react-refresh/only-export-components.
• yarn test — 528 tests passed.
• yarn typecheck and yarn build — passed.
• yarn lint — no errors; 7 unrelated existing warnings remain.
• yarn test:pw theme-contrast.spec.ts app-shell --project=chromium — all 12 Chromium cases passed in the combined browser run.

Firefox was also attempted, including a single-worker retry, but the local Firefox process exits before running tests with Could not find profile folder. No Firefox pass is claimed.

Implemented with AI assistance; all changes were reviewed and validated locally.