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/playback_mode.rs

@@ -8,6 +8,7 @@ pub struct PlaybackModeManagerWrapper(pub Arc<PlaybackModeManager>);
 
 /// Get the current playback mode
 #[tauri::command]
+#[specta::specta]
 pub fn playback_mode_get_current(
     manager: State<'_, PlaybackModeManagerWrapper>,
 ) -> Result<PlaybackMode, String> {
@@ -16,6 +17,7 @@ pub fn playback_mode_get_current(
 
 /// Set the playback mode (internal/testing use)
 #[tauri::command]
+#[specta::specta]
 pub fn playback_mode_set(
     manager: State<'_, PlaybackModeManagerWrapper>,
     mode: PlaybackMode,
@@ -26,6 +28,7 @@ pub fn playback_mode_set(
 
 /// Check if currently transferring between playback modes
 #[tauri::command]
+#[specta::specta]
 pub fn playback_mode_is_transferring(
     manager: State<'_, PlaybackModeManagerWrapper>,
 ) -> Result<bool, String> {
@@ -34,6 +37,7 @@ pub fn playback_mode_is_transferring(
 
 /// Transfer playback from local device to a remote Jellyfin session
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_mode_transfer_to_remote(
     manager: State<'_, PlaybackModeManagerWrapper>,
     session_id: String,
@@ -51,6 +55,7 @@ pub async fn playback_mode_transfer_to_remote(
 /// - current_item_id: The Jellyfin item ID currently playing on remote
 /// - position_ticks: Current playback position in ticks (10,000 ticks = 1ms)
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_mode_transfer_to_local(
     manager: State<'_, PlaybackModeManagerWrapper>,
     current_item_id: String,
@@ -69,6 +74,7 @@ pub async fn playback_mode_transfer_to_local(
 
 /// Get remote session status (for polling position/duration)
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_mode_get_remote_status(
     manager: State<'_, PlaybackModeManagerWrapper>,
     player: State<'_, crate::commands::PlayerStateWrapper>,
@@ -119,7 +125,7 @@ pub async fn playback_mode_get_remote_status(
 }
 
 /// Remote session status for UI updates
-#[derive(serde::Serialize)]
+#[derive(specta::Type, serde::Serialize)]
 #[serde(rename_all = "camelCase")]
 pub struct RemoteSessionStatus {
     pub position: f64,