fix(queue): pin queueServerId on auto-add paths so infinite + radio top-up refs resolve (#930)

* fix(queue): extend server-pin contract to auto-add paths

The infinite-queue top-up and radio top-up paths in nextAction.ts
read state.queueServerId directly inside their set callbacks. When
the queue was populated without a queue-replacing playTrack (single-
track enqueue from a SongRow + button, AdvancedSearch row, etc),
queueServerId stayed null, seedQueueResolver skipped its store-write
under the if (serverId) guard, and the auto-added refs landed with
an empty server key. Every auto-added row rendered as the resolver
placeholder (… / 0:00) until the next time something happened to
bind the server. Same symptom PR #892 fixed for the manual enqueue
surface, just on the auto-add paths.

Extract ensureQueueServerPinned() from the private helper in
queueMutationActions.ts into playbackServer.ts so it can be shared.
Call it before every set callback that appends or splices refs in
nextAction.ts — appendTracksAndPlayFirst, proactive infinite top-up,
proactive radio top-up. Helper returns the pinned canonical key so
the caller does not need a second store read.

Regression coverage in ensureQueueServerPinned.test.ts: pin on null
+ active server, idempotent on already-bound, empty-string fallback
when no active server, canonical-key return value matches what
toQueueItemRefs expects (not the raw auth uuid). Existing
b1QueueServerIdentity.test.ts continues to cover the manual
enqueue surface unchanged.

* docs(release): CHANGELOG for queue auto-top-up placeholder fix (PR #930)

src/utils/playback/playbackServer.ts

@@ -57,6 +57,27 @@ export function bindQueueServerForPlayback(): void {
   usePlayerStore.setState({ queueServerId: canonical });
 }
 
+/**
+ * Bind `queueServerId` via {@link bindQueueServerForPlayback} when it is still
+ * null, then return the (now-bound) server identifier. Call this synchronously
+ * before any state mutation that adds new tracks to the queue.
+ *
+ * Without the pin, refs land with an empty server key, {@link seedQueueResolver}
+ * skips its store-write, and queue rows render as the resolver placeholder
+ * (`…` / 0:00) until something else binds the server (see PR #892). Affects
+ * both the manual enqueue mutations and the auto-add paths (infinite-queue
+ * top-up, radio top-up).
+ *
+ * Idempotent: no-op when already pinned. Returns `''` when no active server is
+ * available to pin (e.g. unit tests without an authed store).
+ */
+export function ensureQueueServerPinned(): string {
+  if (usePlayerStore.getState().queueServerId == null) {
+    bindQueueServerForPlayback();
+  }
+  return usePlayerStore.getState().queueServerId ?? '';
+}
+
 export function clearQueueServerForPlayback(): void {
   usePlayerStore.setState({ queueServerId: null });
 }