Remove theme and locale from instance settings to eliminate duplication and
simplify the codebase. These are user-specific preferences and should only
exist in user settings, not instance-wide settings.

Backend changes:
- Remove theme from InstanceGeneralSetting proto
- Remove locale from InstanceCustomProfile proto
- Update instance service converters to remove theme/locale handling
- Simplify RSS feed to use static locale

Frontend changes:
- Remove theme/locale from instanceStore state
- Create unified initialization flow with clear fallback priority:
  * Theme: user setting → localStorage → system preference
  * Locale: user setting → browser language
- Add applyUserPreferences() to centralize theme/locale application
- Simplify App.tsx by removing redundant state synchronization
- Update all components to use new helper functions:
  * getThemeWithFallback() for theme resolution
  * getLocaleWithFallback() for locale resolution
- Remove theme/locale selectors from instance profile dialog

Theme utilities refactor:
- Organize code into clear sections with JSDoc comments
- Extract localStorage operations into getStoredTheme/setStoredTheme helpers
- Split DOM manipulation into focused functions
- Improve type safety with Theme and ResolvedTheme types
- Reduce code duplication and improve maintainability

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>