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/connectivity.rs

@@ -7,6 +7,7 @@ pub struct ConnectivityMonitorWrapper(pub Arc<tokio::sync::Mutex<ConnectivityMon
 
 /// Check if the server is currently reachable
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_check_server(
     state: State<'_, ConnectivityMonitorWrapper>,
 ) -> Result<bool, String> {
@@ -16,6 +17,7 @@ pub async fn connectivity_check_server(
 
 /// Set the server URL and trigger an immediate check
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_set_server_url(
     url: String,
     state: State<'_, ConnectivityMonitorWrapper>,
@@ -27,6 +29,7 @@ pub async fn connectivity_set_server_url(
 
 /// Get the current connectivity status
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_get_status(
     state: State<'_, ConnectivityMonitorWrapper>,
 ) -> Result<ConnectivityStatus, String> {
@@ -36,6 +39,7 @@ pub async fn connectivity_get_status(
 
 /// Start monitoring connectivity with adaptive polling
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_start_monitoring(
     state: State<'_, ConnectivityMonitorWrapper>,
 ) -> Result<(), String> {
@@ -46,6 +50,7 @@ pub async fn connectivity_start_monitoring(
 
 /// Stop monitoring connectivity
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_stop_monitoring(
     state: State<'_, ConnectivityMonitorWrapper>,
 ) -> Result<(), String> {
@@ -56,6 +61,7 @@ pub async fn connectivity_stop_monitoring(
 
 /// Mark the server as reachable (called after successful API calls)
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_mark_reachable(
     state: State<'_, ConnectivityMonitorWrapper>,
 ) -> Result<(), String> {
@@ -66,6 +72,7 @@ pub async fn connectivity_mark_reachable(
 
 /// Mark the server as unreachable (called after failed API calls)
 #[tauri::command]
+#[specta::specta]
 pub async fn connectivity_mark_unreachable(
     error: Option<String>,
     state: State<'_, ConnectivityMonitorWrapper>,