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/offline.rs

@@ -7,7 +7,7 @@ use tauri::State;
 use super::DatabaseWrapper;
 use crate::storage::db_service::{DatabaseService, Query, QueryParam};
 
-#[derive(Debug, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct OfflineItem {
     pub id: String,
@@ -22,6 +22,7 @@ pub struct OfflineItem {
 
 /// Check if an item is available offline
 #[tauri::command]
+#[specta::specta]
 pub async fn offline_is_available(
     db: State<'_, DatabaseWrapper>,
     item_id: String,
@@ -46,6 +47,7 @@ pub async fn offline_is_available(
 
 /// Get all offline items for a user
 #[tauri::command]
+#[specta::specta]
 pub async fn offline_get_items(
     db: State<'_, DatabaseWrapper>,
     user_id: String,
@@ -84,6 +86,7 @@ pub async fn offline_get_items(
 
 /// Search offline items
 #[tauri::command]
+#[specta::specta]
 pub async fn offline_search(
     db: State<'_, DatabaseWrapper>,
     user_id: String,