Fix download + casting breakages from the specta migration

Casting: restore camelCase serialization on SessionInfo/NowPlayingItem/PlayState
(container rename_all = "camelCase" + per-field PascalCase serde aliases so Jellyfin
PascalCase still deserializes). Bindings and the existing frontend are camelCase
again — casting works with no frontend session changes.

Downloads: migrate DownloadButton.svelte and downloads.ts to the typed
commands.downloadItemAndStart / downloadItem / downloadVideo wrappers (request
objects), matching the bundled backend signatures.

Tooling:
- Enable tauri-specta ErrorHandlingMode::Throw so commands.* return Promise<T>
  and throw (drop-in for invoke()).
- Disambiguate specta name collisions: player MediaItem/MediaSource ->
  PlayerMediaItem/PlayerMediaSource, auth ServerInfo -> AuthServerInfo.
- Regenerate bindings.ts; svelte-check passes (0 errors).

src-tauri/src/jellyfin/client.rs

@@ -382,71 +382,102 @@ fn default_true() -> bool {
 
 /// Session information from Jellyfin
 #[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
-#[serde(rename_all = "PascalCase")]
+#[serde(rename_all = "camelCase")]
 pub struct SessionInfo {
     #[serde(default)]
+    #[serde(alias = "Id")]
     pub id: Option<String>,
     #[serde(default)]
+    #[serde(alias = "UserId")]
     pub user_id: Option<String>,
     #[serde(default)]
+    #[serde(alias = "UserName")]
     pub user_name: Option<String>,
     #[serde(default)]
+    #[serde(alias = "Client")]
     pub client: Option<String>,
     #[serde(default)]
+    #[serde(alias = "DeviceName")]
     pub device_name: Option<String>,
     #[serde(default)]
+    #[serde(alias = "DeviceId")]
     pub device_id: Option<String>,
     #[serde(default)]
+    #[serde(alias = "ApplicationVersion")]
     pub application_version: Option<String>,
     #[serde(default)]
+    #[serde(alias = "IsActive")]
     pub is_active: Option<bool>,
     #[serde(default)]
+    #[serde(alias = "SupportsMediaControl")]
     pub supports_media_control: Option<bool>,
     #[serde(default = "default_true")]
+    #[serde(alias = "SupportsRemoteControl")]
     pub supports_remote_control: bool,
     #[serde(default)]
+    #[serde(alias = "NowPlayingItem")]
     pub now_playing_item: Option<NowPlayingItem>,
     #[serde(default)]
+    #[serde(alias = "PlayState")]
     pub play_state: Option<PlayState>,
     #[serde(default)]
+    #[serde(alias = "PlayableMediaTypes")]
     pub playable_media_types: Option<Vec<String>>,
     #[serde(default)]
+    #[serde(alias = "SupportedCommands")]
     pub supported_commands: Option<Vec<String>>,
 }
 
 #[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
-#[serde(rename_all = "PascalCase")]
+#[serde(rename_all = "camelCase")]
 pub struct NowPlayingItem {
+    #[serde(alias = "Id")]
     pub id: Option<String>,
+    #[serde(alias = "Name")]
     pub name: Option<String>,
+    #[serde(alias = "RunTimeTicks")]
     pub run_time_ticks: Option<i64>,
+    #[serde(alias = "Album")]
     pub album: Option<String>,
+    #[serde(alias = "AlbumId")]
     pub album_id: Option<String>,
+    #[serde(alias = "AlbumArtist")]
     pub album_artist: Option<String>,
+    #[serde(alias = "Artists")]
     pub artists: Option<Vec<String>>,
+    #[serde(alias = "ImageTags")]
     pub image_tags: Option<std::collections::HashMap<String, String>>,
+    #[serde(alias = "PrimaryImageTag")]
     pub primary_image_tag: Option<String>,
+    #[serde(alias = "AlbumPrimaryImageTag")]
     pub album_primary_image_tag: Option<String>,
     #[serde(rename = "Type")]
     pub item_type: Option<String>,
 }
 
 #[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
-#[serde(rename_all = "PascalCase")]
+#[serde(rename_all = "camelCase")]
 pub struct PlayState {
     #[serde(default)]
+    #[serde(alias = "PositionTicks")]
     pub position_ticks: Option<i64>,
     #[serde(default)]
+    #[serde(alias = "CanSeek")]
     pub can_seek: Option<bool>,
     #[serde(default)]
+    #[serde(alias = "IsPaused")]
     pub is_paused: Option<bool>,
     #[serde(default)]
+    #[serde(alias = "IsMuted")]
     pub is_muted: Option<bool>,
     #[serde(default)]
+    #[serde(alias = "VolumeLevel")]
     pub volume_level: Option<i32>,
     #[serde(default)]
+    #[serde(alias = "RepeatMode")]
     pub repeat_mode: Option<String>,
     #[serde(default)]
+    #[serde(alias = "ShuffleMode")]
     pub shuffle_mode: Option<String>,
 }