refactor: migrate JS/MJS sources and scripts to TypeScript

Migrate all remaining migratable .js/.mjs files to .ts with strong typing:
- constants/MediaTypes: as const + derived MediaType union
- utils/profiles/{download,subtitles,trackplayer}: typed against
  @jellyfin/sdk DeviceProfile/SubtitleProfile/CodecProfile models;
  native.ts now validates its profile with `satisfies DeviceProfile`
- trackplayer.d.ts removed (superseded by real TS implementation)
- index.js -> index.ts (entry point, "main" is extension-less)
- scripts/{typecheck,check-i18n-keys,detect-duplicate-issue} -> .ts,
  all run via bun (typecheck switched from node to bun)

Remove scripts/symlink-native-dirs.js: dead since 446439c2 (2025-02-28)
when its only reference (prebuild:tv-new) was dropped; superseded by
`expo prebuild --clean` + cross-env EXPO_TV. Drop the matching
.gitignore relics (/iostv, /iosmobile, /androidmobile, /androidtv).

Document tooling-required .js exceptions (babel/metro/react-native/
tailwind configs) in CLAUDE.md and copilot-instructions.md so code
review guidelines stop flagging them.

.github/copilot-instructions.md

@@ -42,7 +42,7 @@ and provides seamless media streaming with offline capabilities and Chromecast s
 
 ## Coding Standards
 
-- Use TypeScript for ALL files (no .js files)
+- Use TypeScript for ALL files (no .js files). Tooling-required exceptions: `babel.config.js`, `metro.config.js`, `react-native.config.js`, `tailwind.config.js` (their loaders cannot parse TypeScript)
 - Use descriptive English names for variables, functions, and components
 - Prefer functional React components with hooks
 - Use Jotai atoms for global state management