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- trackplayer.d.ts removed (superseded by real TS implementation)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when its only reference (prebuild:tv-new) was dropped; superseded by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

utils/profiles/native.ts

@@ -3,6 +3,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
+import type { DeviceProfile } from "@jellyfin/sdk/lib/generated-client/models";
 import { Platform } from "react-native";
 import MediaTypes from "../../constants/MediaTypes";
 import { getSubtitleProfiles } from "./subtitles";
@@ -193,7 +194,7 @@ export const generateDeviceProfile = (options: ProfileOptions = {}) => {
       },
     ],
     SubtitleProfiles: getSubtitleProfiles(),
-  };
+  } satisfies DeviceProfile;
 
   return profile;
 };