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/jellyfin/client.rs

@@ -381,8 +381,8 @@ fn default_true() -> bool {
 }
 
 /// Session information from Jellyfin
-#[derive(Debug, Clone, Deserialize, serde::Serialize)]
-#[serde(rename_all(deserialize = "PascalCase", serialize = "camelCase"))]
+#[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
+#[serde(rename_all = "PascalCase")]
 pub struct SessionInfo {
     #[serde(default)]
     pub id: Option<String>,
@@ -414,8 +414,8 @@ pub struct SessionInfo {
     pub supported_commands: Option<Vec<String>>,
 }
 
-#[derive(Debug, Clone, Deserialize, serde::Serialize)]
-#[serde(rename_all(deserialize = "PascalCase", serialize = "camelCase"))]
+#[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
+#[serde(rename_all = "PascalCase")]
 pub struct NowPlayingItem {
     pub id: Option<String>,
     pub name: Option<String>,
@@ -431,8 +431,8 @@ pub struct NowPlayingItem {
     pub item_type: Option<String>,
 }
 
-#[derive(Debug, Clone, Deserialize, serde::Serialize)]
-#[serde(rename_all(deserialize = "PascalCase", serialize = "camelCase"))]
+#[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
+#[serde(rename_all = "PascalCase")]
 pub struct PlayState {
     #[serde(default)]
     pub position_ticks: Option<i64>,