feat(radio): wire radio + similarity into the queue (A11/B6, §6.5)

"Start radio" in the track menu seeds the queue from a track; RadioController
(headless, in AppShell) tops it up via /radio/next as it drains. QueuePanel's
radio scaffolding is now live: the exploration slider drives familiar<->new and
each entry shows its reason ("why this is playing"). Adds the radio endpoints
(start/next/similar), radio context + actions on the queue slice, and the pick->
entry mapper. Degrades to metadata picks while ML is offline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/player/RadioController.tsx

@@ -0,0 +1,51 @@
+import { useEffect, useRef } from 'react';
+import { useAppDispatch, useAppSelector } from '../../hooks/useAppDispatch';
+import { useRadioNextMutation } from '../../api/endpoints/radio';
+import { appendToQueue } from '../../store/slices/queue';
+import { pickToEntry } from '../../lib/radio';
+
+// Prefetch more radio tracks once the current position is within this many of
+// the end of the queue.
+const LOOKAHEAD = 3;
+
+/**
+ * Headless controller: while a radio drives the queue, top it up as it drains
+ * (§6.5 / B6 infinite feed). Mounted once in AppShell. A failed prefetch just
+ * stops the feed — playback of what's already queued is unaffected.
+ */
+export function RadioController() {
+  const dispatch = useAppDispatch();
+  const radio = useAppSelector((s) => s.queue.radio);
+  const currentIndex = useAppSelector((s) => s.queue.currentIndex);
+  const entries = useAppSelector((s) => s.queue.entries);
+  const [radioNext] = useRadioNextMutation();
+  const loadingRef = useRef(false);
+
+  useEffect(() => {
+    if (!radio) return;
+    const nearEnd = currentIndex >= entries.length - LOOKAHEAD;
+    if (!nearEnd || loadingRef.current) return;
+
+    loadingRef.current = true;
+    void (async () => {
+      try {
+        const res = await radioNext({
+          seedTrackId: radio.seedTrackId ?? undefined,
+          fromLikes: radio.fromLikes,
+          exploration: radio.exploration,
+          count: 10,
+          excludeIds: entries.map((e) => e.trackId),
+        }).unwrap();
+        if (res.picks.length > 0) {
+          dispatch(appendToQueue(res.picks.map(pickToEntry)));
+        }
+      } catch {
+        // best-effort — leave the queue as-is
+      } finally {
+        loadingRef.current = false;
+      }
+    })();
+  }, [radio, currentIndex, entries, radioNext, dispatch]);
+
+  return null;
+}