Wire dark mode sidebar toggle

[version-1]

Wire the dark-mode Sidebar toggle

Summary

Connect the existing Sidebar switch to layout-level theme state. An explicit
user selection is persisted locally; until then, the UI follows the operating
system color-scheme preference.

主な変更点:

• Add a layout theme hook that applies the existing dark token set.
• Persist explicit dark / light choices under tasq.theme.
• Keep the Sidebar switch controlled and cover persistence and OS-following behavior.

チケットへのリンク

Tasq issue #215

やったこと

• Saved theme values take precedence over prefers-color-scheme.
• Without a saved value, matchMedia changes update the theme in real time.
• Toggling the switch stores the selected theme and stops OS-following.

動作確認

• npm test (104 tests)
• npm run typecheck
• npm run build

レビュー & 動作確認 チェックリスト

• Persistence - Reload uses the explicitly saved theme.
• System preference - No saved theme follows OS dark-mode changes.
• Scope - Existing language and refresh-interval persistence remain unchanged.

推奨テスト計画:

1. Remove tasq.theme from localStorage and change the OS preference.
2. Toggle Dark mode from the Sidebar and reload.
3. Confirm language and refresh interval settings still persist.

その他気になることや相談ごと

The two-state switch intentionally does not indicate whether its current value
comes from the system preference or an explicit user selection.