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- trackplayer.d.ts removed (superseded by real TS implementation)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when its only reference (prebuild:tv-new) was dropped; superseded by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.

package.json

@@ -17,13 +17,13 @@
     "ios:unsigned-build": "cross-env EXPO_TV=0 bun scripts/ios/build-ios.ts --production",
     "ios:unsigned-build:tv": "cross-env EXPO_TV=1 bun scripts/ios/build-ios.ts --production",
     "prepare": "husky",
-    "typecheck": "node scripts/typecheck.js",
+    "typecheck": "bun scripts/typecheck.ts",
     "check": "biome check . --max-diagnostics 1000",
     "lint": "biome check --write --unsafe --max-diagnostics 1000",
     "format": "biome format --write .",
     "doctor": "expo-doctor",
-    "i18n:check": "bun scripts/check-i18n-keys.mjs",
-    "i18n:fix-unused": "bun scripts/check-i18n-keys.mjs --fix-unused",
+    "i18n:check": "bun scripts/check-i18n-keys.ts",
+    "i18n:fix-unused": "bun scripts/check-i18n-keys.ts --fix-unused",
     "test": "bun run typecheck && bun run lint && bun run format && bun run i18n:check && bun run doctor",
     "postinstall": "patch-package"
   },