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/stores/downloads.ts

@@ -2,6 +2,7 @@
 // TRACES: UR-011, UR-013, UR-018 | DR-015, DR-017
 import { writable, derived, get } from 'svelte/store';
 import { invoke } from '@tauri-apps/api/core';
+import { commands } from '$lib/api/bindings';
 import { listen, type UnlistenFn } from '@tauri-apps/api/event';
 
 // Event listener state
@@ -150,15 +151,16 @@ function createDownloadsStore() {
 		): Promise<number> {
 			try {
 				console.log('📥 downloadItem called:', { itemId, userId, filePath, itemName, artistName, albumName });
-				const downloadId = await invoke<number>('download_item', {
+				const downloadId = await commands.downloadItem({
 					itemId,
 					userId,
 					filePath,
-					mimeType,
-					priority,
-					itemName,
-					artistName,
-					albumName
+					mimeType: mimeType ?? null,
+					priority: priority ?? null,
+					itemName: itemName ?? null,
+					artistName: artistName ?? null,
+					albumName: albumName ?? null,
+					expectedSize: null
 				});
 				console.log('   Got download ID from backend:', downloadId);
 
@@ -222,18 +224,18 @@ function createDownloadsStore() {
 					qualityPreset,
 					seriesName
 				});
-				const downloadId = await invoke<number>('download_video', {
+				const downloadId = await commands.downloadVideo({
 					itemId,
 					userId,
 					filePath,
-					mimeType,
-					priority,
-					itemName,
-					qualityPreset,
-					seriesName,
-					seasonName,
-					episodeNumber,
-					seasonNumber
+					mimeType: mimeType ?? null,
+					priority: priority ?? null,
+					itemName: itemName ?? null,
+					qualityPreset: qualityPreset ?? null,
+					seriesName: seriesName ?? null,
+					seasonName: seasonName ?? null,
+					episodeNumber: episodeNumber ?? null,
+					seasonNumber: seasonNumber ?? null
 				});
 				console.log('   Got download ID from backend:', downloadId);