E frontend reconciliation (1/2): types.ts sources from bindings; backend field fixes

- types.ts now re-exports wire types from generated bindings (single source of
  truth); keeps frontend-only unions (ItemType/LibraryType/PersonType/
  SessionCommand) and aliases Session = SessionInfo.
- Backend: MediaItem.runtime_ticks serializes as runTimeTicks (matches frontend,
  fixes a latent undefined-read bug); ArtistItem serializes camelCase id/name
  (PascalCase aliases retained for Jellyfin deserialize).
- player.ts: normalize remote NowPlayingItem -> MediaItem in mergedMedia so
  display components treat local/remote items uniformly.
- Reduces svelte-check errors 141 -> 70 (remaining: nullable guards + played->isPlayed).

src/lib/stores/player.ts

@@ -9,7 +9,8 @@
  */
 
 import { writable, derived } from "svelte/store";
-import type { MediaItem } from "$lib/api/types";
+import type { MediaItem, ItemType } from "$lib/api/types";
+import type { NowPlayingItem } from "$lib/api/bindings";
 import { isRemoteMode } from "./playbackMode";
 import { selectedSession } from "./sessions";
 import { ticksToSeconds } from "$lib/utils/playbackUnits";
@@ -161,13 +162,36 @@ export const isMuted = derived(player, ($p) => $p.muted);
 /**
  * Merged media item - prefers remote session when in remote mode
  */
-export const mergedMedia = derived(
+/**
+ * Normalize a remote session's NowPlayingItem into the MediaItem shape the UI
+ * renders, so display components can treat local and remote items uniformly.
+ * (NowPlayingItem has `album`/`artists` but no `albumName`/`artistItems`; the UI
+ * falls back to the `artists` string list when `artistItems` is absent.)
+ */
+function nowPlayingToMediaItem(npi: NowPlayingItem): MediaItem {
+  return {
+    id: npi.id ?? "",
+    name: npi.name ?? "",
+    type: (npi.Type ?? "Audio") as ItemType,
+    serverId: "",
+    albumName: npi.album,
+    albumId: npi.albumId,
+    artists: npi.artists,
+    primaryImageTag: npi.primaryImageTag ?? npi.albumPrimaryImageTag,
+    runTimeTicks: npi.runTimeTicks,
+  } as MediaItem;
+}
+
+export const mergedMedia = derived<
+  [typeof isRemoteMode, typeof selectedSession, typeof currentMedia],
+  MediaItem | null
+>(
   [isRemoteMode, selectedSession, currentMedia],
   ([$isRemote, $session, $local]) => {
     if ($isRemote && $session?.nowPlayingItem) {
-      return $session.nowPlayingItem;
+      return nowPlayingToMediaItem($session.nowPlayingItem);
     }
-    return $local;
+    return $local ?? null;
   }
 );