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/playback_mode/mod.rs

@@ -12,7 +12,7 @@ use crate::jellyfin::JellyfinClient;
 use crate::player::{PlayerController, QueueContext};
 
 /// Playback mode - local device, remote session, or idle
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 #[serde(tag = "type", rename_all = "lowercase")]
 pub enum PlaybackMode {
     Local,