Reduce code duplication and update README

- Extract shared utilities (HIDDEN_FILES, VIDEO_EXTENSIONS, fileApiUrl,
  thumbnailApiUrl, findFile) into new src/lib/media-utils.ts, removing
  identical copies from games.ts, movies.ts, tv.ts, files.ts, and scanner.ts
- Add comment in files.ts clarifying why its VIDEO_EXTENSIONS set intentionally
  differs from the media library set (web-playable formats for the mixed browser)
- Rewrite README to reflect the current feature set: Movies/TV libraries, auth
  system, tag system, background scanner, updated project structure, folder
  conventions for all four library types, and a complete API reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/scanner.ts

@@ -8,8 +8,8 @@ import { scanMoviesLibrary } from './movies'
 import { scanTvLibrary, scanTvSeasons, scanTvEpisodes } from './tv'
 import { scanGamesLibrary } from './games'
 import { getThumbnailPath } from './thumbnails'
+import { VIDEO_EXTENSIONS } from './media-utils'
 
-const VIDEO_EXTENSIONS = new Set(['.mkv', '.mp4', '.avi', '.mov', '.m4v', '.wmv', '.ts', '.m2ts'])
 const IMAGE_EXTENSIONS = new Set(['.jpg', '.jpeg', '.png', '.gif', '.webp', '.bmp'])
 
 let scanRunning = false