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/store/slices/queue.ts

@@ -15,6 +15,15 @@ export interface QueueEntry {
   albumTitle: string;
   durationMs: number;
   albumArtUrl?: string;
+  /** Radio reason code (`ml`/`similar`/`from_likes`/`discover`) — §6.5. */
+  reason?: string;
+}
+
+/** Active-radio context: what seeds the infinite feed + the exploration bias. */
+export interface RadioContext {
+  seedTrackId: string | null;
+  fromLikes: boolean;
+  exploration: number;
 }
 
 export interface QueueState {
@@ -25,6 +34,8 @@ export interface QueueState {
   sourceName: string | null;
   shuffle: boolean;
   loop: boolean;
+  /** Non-null while a radio drives the queue (enables infinite loading). */
+  radio: RadioContext | null;
 }
 
 export const queueInitialState: QueueState = {
@@ -35,6 +46,7 @@ export const queueInitialState: QueueState = {
   sourceName: null,
   shuffle: false,
   loop: false,
+  radio: null,
 };
 
 export const queueSlice = createSlice({
@@ -56,6 +68,27 @@ export const queueSlice = createSlice({
       state.source = action.payload.source;
       state.sourceId = action.payload.sourceId ?? null;
       state.sourceName = action.payload.sourceName ?? null;
+      state.radio = null; // a manual/album/… queue ends any active radio
+    },
+    startRadioQueue(
+      state,
+      action: PayloadAction<{ entries: QueueEntry[]; radio: RadioContext }>,
+    ) {
+      state.entries = action.payload.entries;
+      state.currentIndex = 0;
+      state.source = 'radio';
+      state.sourceId = null;
+      state.sourceName = null;
+      state.radio = action.payload.radio;
+    },
+    appendToQueue(state, action: PayloadAction<QueueEntry[]>) {
+      state.entries.push(...action.payload);
+    },
+    setExploration(state, action: PayloadAction<number>) {
+      if (state.radio) state.radio.exploration = action.payload;
+    },
+    stopRadio(state) {
+      state.radio = null;
     },
     addToQueue(state, action: PayloadAction<QueueEntry>) {
       state.entries.push(action.payload);
@@ -102,6 +135,7 @@ export const queueSlice = createSlice({
     clearQueue(state) {
       state.entries = [];
       state.currentIndex = -1;
+      state.radio = null;
     },
     toggleShuffle(state) {
       state.shuffle = !state.shuffle;
@@ -114,6 +148,10 @@ export const queueSlice = createSlice({
 
 export const {
   setQueue,
+  startRadioQueue,
+  appendToQueue,
+  setExploration,
+  stopRadio,
   addToQueue,
   addNextInQueue,
   playNow,