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/lib/components/library/DownloadButton.svelte

@@ -2,6 +2,7 @@
   import { downloads } from "$lib/stores/downloads";
   import { auth } from "$lib/stores/auth";
   import { invoke } from "@tauri-apps/api/core";
+  import { commands } from "$lib/api/bindings";
   import DownloadButtonCore from "./DownloadButtonCore.svelte";
   import type { DownloadState } from "./DownloadButtonCore.svelte";
 
@@ -84,14 +85,14 @@
           console.log("   Target directory:", targetDir);
 
           // Queue and start download in single atomic operation
-          const downloadId = await invoke<number>("download_item_and_start", {
+          const downloadId = await commands.downloadItemAndStart({
             itemId,
             userId,
             streamUrl,
             targetDir,
-            itemName: itemName || undefined,
-            artistName: artistName || undefined,
-            albumName: albumName || undefined,
+            itemName: itemName || null,
+            artistName: artistName || null,
+            albumName: albumName || null,
           });
           console.log("   Download queued and started with ID:", downloadId);