feat: implement library.rs, types.ts, api.ts, and extract CSS

- library.rs: full video library management (1948 lines, 10 tests)
  folder scanning, progress tracking, playlists, subtitle integration,
  background duration scanning
- types.ts: all TypeScript interfaces for Tauri command responses
- api.ts: typed wrappers for all 26 Tauri invoke commands
- 6 CSS files extracted from Python HTML into src/styles/

src/main.ts

@@ -1 +1,8 @@
-console.log("TutorialDock frontend loaded");
+import './styles/main.css';
+import './styles/player.css';
+import './styles/playlist.css';
+import './styles/panels.css';
+import './styles/components.css';
+import './styles/animations.css';
+
+console.log('TutorialDock frontend loaded');