feat: add dark/light theme toggle and UI improvements

[Po1nt9]

Summary

This PR adds a dark/light theme toggle feature and several UI improvements to LLM Wiki.

Features

1. Dark/Light Theme Toggle

   • Added theme selection in Settings > Interface section
   • Three options: Light, Dark, System (follows OS preference)
   • Real-time preview - theme changes apply immediately when clicked
   • Theme preference persists to local storage
   • Auto-applies saved theme on app startup to prevent flash

2. Welcome Screen Centering

   • Fixed welcome screen to be fully vertically centered using h-screen

3. Dark Theme Background Optimization

   • Added subtle blue tint to dark theme background for better visual depth
   • Adjusted card, sidebar, and accent colors for consistency

4. Test Fixes

   • Fixed pre-existing test failures in ingest-queue.integration.test.ts
   • Fixed path separator issues in ingest-source-path-collision.test.ts
   • All 1214 unit tests now pass

Files Changed

• src/components/settings/settings-types.ts - Added theme type to SettingsDraft
• src/components/settings/settings-view.tsx - Added theme state management and applyTheme function
• src/components/settings/sections/interface-section.tsx - Added theme toggle UI
• src/lib/project-store.ts - Added saveTheme/loadTheme persistence functions
• src/main.tsx - Added theme initialization on app startup
• src/index.css - Optimized dark theme CSS variables
• src/i18n/en.json - Added theme-related translations
• src/i18n/zh.json - Added theme-related translations
• src/components/project/welcome-screen.tsx - Fixed vertical centering
• src/components/graph/graph-view.tsx - Updated graph background to use theme variable
• src/lib/ingest-queue.integration.test.ts - Fixed JSON parse timing issues
• src/lib/ingest-source-path-collision.test.ts - Fixed Windows path separator issues

Test plan

• TypeScript type check passes
• Build succeeds
• All 1214 unit tests pass
• Theme toggle works in real-time (Light/Dark/System)
• Theme persists across app restarts
• Welcome screen centered correctly
• Dark theme background looks good

Screenshots

Theme toggle is located in Settings > Interface section, below the UI Language selector.