fix(web): missing files show greyed in track lists and never enter the queue (#5418)

TrackRef gains the unavailable flag the server already sends on album
detail and search. TrackRow greys such a row with '· File missing' and
hides like, radio, add and the menu, as playlist rows already do. Every
player-store entry point (playQueue, enqueue, play next, and a refetched
mix) drops missing files; playQueue keeps the chosen track as the start.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/api/types.ts

@@ -41,6 +41,10 @@ export type TrackRef = {
   disc_number?: number;
   duration_sec: number;
   stream_url: string;
+  // #5418: the file is gone from the library (missing_since is set). Sent on
+  // direct lookups such as album detail and search; lists that pick tracks
+  // for playback leave missing files out instead.
+  unavailable?: boolean;
 };
 
 export type ArtistDetail = ArtistRef & {