Fix download + casting breakages from the specta migration

Casting: restore camelCase serialization on SessionInfo/NowPlayingItem/PlayState
(container rename_all = "camelCase" + per-field PascalCase serde aliases so Jellyfin
PascalCase still deserializes). Bindings and the existing frontend are camelCase
again — casting works with no frontend session changes.

Downloads: migrate DownloadButton.svelte and downloads.ts to the typed
commands.downloadItemAndStart / downloadItem / downloadVideo wrappers (request
objects), matching the bundled backend signatures.

Tooling:
- Enable tauri-specta ErrorHandlingMode::Throw so commands.* return Promise<T>
  and throw (drop-in for invoke()).
- Disambiguate specta name collisions: player MediaItem/MediaSource ->
  PlayerMediaItem/PlayerMediaSource, auth ServerInfo -> AuthServerInfo.
- Regenerate bindings.ts; svelte-check passes (0 errors).

src-tauri/src/lib.rs

@@ -351,6 +351,9 @@ fn create_player_backend(
 /// bindings-export test so the TypeScript bindings always match the handler.
 fn specta_builder() -> Builder<tauri::Wry> {
     Builder::<tauri::Wry>::new()
+        // Throw on error so generated `commands.*` return Promise<T> and throw,
+        // matching the existing frontend's invoke() try/catch convention.
+        .error_handling(tauri_specta::ErrorHandlingMode::Throw)
         .commands(tauri_specta::collect_commands![
             // Player commands
             player_play_item,