refactor(player): E.24 — extract radio session + infinite-queue guards (#588)

Cluster of four small mutables in two modules:

  - `src/store/radioSessionState.ts` — `radioFetching` (concurrent
    fetch guard) + `currentRadioArtistId` (seed artist that survives
    track advances) + `radioSessionSeenIds` (dedupe set including
    HISTORY_KEEP-evicted entries, fixes issue #500).
  - `src/store/infiniteQueueState.ts` — `infiniteQueueFetching`
    concurrent fetch guard.

Sed-driven bulk rewrite for the >35 direct-access sites. The four
`= new Set()` resets become `clearRadioSessionSeenIds()` calls and
the `setRadioArtistId` / `enqueueRadio` actions read through
`getCurrentRadioArtistId()` instead of touching the mutable directly.

15 focused tests across the two modules.

playerStore 2867 → 2865 LOC.

src/store/infiniteQueueState.test.ts

@@ -0,0 +1,29 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import {
+  _resetInfiniteQueueStateForTest,
+  isInfiniteQueueFetching,
+  setInfiniteQueueFetching,
+} from './infiniteQueueState';
+
+afterEach(() => {
+  _resetInfiniteQueueStateForTest();
+});
+
+describe('infiniteQueueFetching', () => {
+  it('starts false', () => {
+    expect(isInfiniteQueueFetching()).toBe(false);
+  });
+
+  it('round-trips through set/get', () => {
+    setInfiniteQueueFetching(true);
+    expect(isInfiniteQueueFetching()).toBe(true);
+    setInfiniteQueueFetching(false);
+    expect(isInfiniteQueueFetching()).toBe(false);
+  });
+
+  it('_resetInfiniteQueueStateForTest resets to false', () => {
+    setInfiniteQueueFetching(true);
+    _resetInfiniteQueueStateForTest();
+    expect(isInfiniteQueueFetching()).toBe(false);
+  });
+});