Support light/dark/system themes and remember user preference (#165)

[kyoungholee]

Summary

• Added full dark mode support alongside the existing light theme.
• Introduced a theme mode switcher (Light / Dark / System) in the Settings → Preferences page.
• Persisted the user’s theme choice so it is remembered across sessions and falls back to the system setting by default.

Changes

• Extended color-schemes.ts with a dedicated dark palette.
• Added ThemeModeContext to manage theme preference (light, dark, system) and resolve the actual mode.
• Persisted the theme preference via shared cache under the themeMode key.
• Wrapped the app with ThemeModeProvider so the resolved mode is applied globally.
• Updated PreferenceView to expose an Appearance section with a 3-way toggle and made styling use theme tokens instead of hard-coded colors where relevant.

Behavior

• Default behavior follows the system prefers-color-scheme setting.
• If the user selects Light or Dark explicitly, that choice overrides the system setting.
• The selected theme is stored and restored on subsequent visits.

Testing

• Manually toggled between Light / Dark / System in Settings and verified:
  • Theme changes immediately across the app.
  • Preference is preserved after page reload.
  • Colors adapt correctly on key screens (Home, Login, Settings).