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/settings.rs

@@ -3,7 +3,7 @@
 use serde::{Deserialize, Serialize};
 
 /// Volume normalization levels matching Spotify's presets
-#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Default)]
+#[derive(specta::Type, Debug, Clone, Copy, PartialEq, Serialize, Deserialize, Default)]
 #[serde(rename_all = "lowercase")]
 pub enum VolumeLevel {
     /// Louder output (-11 LUFS)
@@ -27,7 +27,7 @@ impl VolumeLevel {
 }
 
 /// Audio playback settings
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct AudioSettings {
     /// Crossfade duration in seconds (0 = disabled, max 12)
@@ -60,7 +60,7 @@ impl AudioSettings {
 }
 
 /// Video playback settings
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct VideoSettings {
     /// Enable auto-play of next episode (with countdown)