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/auth/mod.rs

@@ -12,6 +12,7 @@ pub use session_verifier::SessionVerifier;
 /// Server information returned from Jellyfin
 #[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
+#[specta(rename = "AuthServerInfo")]
 pub struct ServerInfo {
     pub name: String,
     pub version: String,