Workstream E (backend): wire tauri-specta — annotate commands, derive Type, generate-ready Builder

- Add #[specta::specta] to all 201 #[tauri::command] functions.
- Derive specta::Type on all IPC DTOs (repository/types, settings, player/storage/
  download command DTOs, player enums, jellyfin SessionInfo/NowPlayingItem/PlayState,
  ThumbnailCacheStats, DownloadInfo, CacheConfig, etc.).
- Replace tauri::generate_handler! with a tauri_specta::Builder + collect_commands!
  in lib.rs (exports bindings.ts in debug builds).

Two contract changes required by specta constraints (frontend migration follows):
- specta caps command arity at 10 args: download_item_and_start / download_item /
  download_video now take a single request struct (params bundled, body unchanged
  via destructuring).
- specta can't parse split serde rename_all: SessionInfo/NowPlayingItem/PlayState
  switched to rename_all = "PascalCase" (Jellyfin deserialization preserved; these
  now serialize PascalCase to the frontend).

cargo check --lib is clean (0 errors). Frontend migration to bindings.ts is the next step.

src-tauri/src/commands/player/remote.rs

@@ -9,6 +9,7 @@ use super::PlayerStateWrapper;
 
 /// Play items on a remote Jellyfin session (casting)
 #[tauri::command]
+#[specta::specta]
 pub async fn remote_play_on_session(
     player: State<'_, PlayerStateWrapper>,
     session_id: String,
@@ -36,6 +37,7 @@ pub async fn remote_play_on_session(
 
 /// Send a playback command to a remote session
 #[tauri::command]
+#[specta::specta]
 pub async fn remote_send_command(
     player: State<'_, PlayerStateWrapper>,
     session_id: String,
@@ -59,6 +61,7 @@ pub async fn remote_send_command(
 
 /// Seek on a remote session
 #[tauri::command]
+#[specta::specta]
 pub async fn remote_session_seek(
     player: State<'_, PlayerStateWrapper>,
     session_id: String,
@@ -82,6 +85,7 @@ pub async fn remote_session_seek(
 
 /// Set volume on a remote session
 #[tauri::command]
+#[specta::specta]
 pub async fn remote_session_set_volume(
     player: State<'_, PlayerStateWrapper>,
     session_id: String,
@@ -105,6 +109,7 @@ pub async fn remote_session_set_volume(
 
 /// Toggle mute on a remote session
 #[tauri::command]
+#[specta::specta]
 pub async fn remote_session_toggle_mute(
     player: State<'_, PlayerStateWrapper>,
     session_id: String,