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/styles/animations.css

@@ -0,0 +1 @@
+/* No standalone animations — included in main.css */