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/api/types.ts

@@ -192,6 +192,20 @@ export interface DuplicateGroup {
   tracks: Track[];
 }
 
+/** One radio/similarity pick with its reason code (§6.5). `reason`: `ml` |
+ * `similar` | `from_likes` | `discover` — localized client-side. */
+export interface RadioPick {
+  track: Track;
+  reason: string;
+}
+
+/** Radio / similar response: where the picks came from + the picks. `source`:
+ * `ml` (recommender) or `metadata` (fallback heuristics). */
+export interface RadioResult {
+  source: string;
+  picks: RadioPick[];
+}
+
 /** Cached lyrics for a track (§6.7). `not_found` is a normal state, not an
  * error. `synced` is raw LRC (client parses timestamps); `plain` is fallback. */
 export interface Lyrics {