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/auth/mod.rs

@@ -10,7 +10,7 @@ use crate::connectivity::ConnectivityMonitor;
 pub use session_verifier::SessionVerifier;
 
 /// Server information returned from Jellyfin
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct ServerInfo {
     pub name: String,
@@ -21,7 +21,7 @@ pub struct ServerInfo {
 }
 
 /// User information
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct User {
     pub id: String,
@@ -31,7 +31,7 @@ pub struct User {
 }
 
 /// Authentication result
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct AuthResult {
     pub user: User,
@@ -40,7 +40,7 @@ pub struct AuthResult {
 }
 
 /// Active session for restoration
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct Session {
     pub user_id: String,
@@ -55,7 +55,7 @@ pub struct Session {
 
 // Jellyfin API response types (PascalCase from server)
 
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "PascalCase")]
 struct PublicSystemInfo {
     server_name: String,
@@ -63,7 +63,7 @@ struct PublicSystemInfo {
     id: String,
 }
 
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "PascalCase")]
 struct AuthenticateByNameResponse {
     user: JellyfinUser,
@@ -71,7 +71,7 @@ struct AuthenticateByNameResponse {
     server_id: String,
 }
 
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "PascalCase")]
 struct JellyfinUser {
     id: String,