refactor(player): M0 — extract pure helpers from playerStore.ts (#554)

Moves four self-contained helpers into src/utils/, each with co-located
characterization tests. playerStore re-exports them for the ~30 existing
call sites; Phase E will migrate those imports.

  - shuffleArray              (Fisher-Yates, generic)
  - resolveReplayGainDb       (track/album/auto mode resolution)
  - songToTrack               (Subsonic -> Track shape)
  - buildInfiniteQueueCandidates  (Instant-Mix top-up source)

playerStore.ts: 3732 -> 3618 LOC (-114).

src/utils/shuffleArray.test.ts

@@ -0,0 +1,51 @@
+/**
+ * Pure-helper characterization for `shuffleArray` (Fisher-Yates).
+ *
+ * Originally lived in `playerStore.ts`; extracted in M0 of the frontend
+ * refactor (2026-05-12).
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { shuffleArray } from './shuffleArray';
+
+describe('shuffleArray', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('does not mutate the input array', () => {
+    const input = [1, 2, 3, 4, 5];
+    const snapshot = [...input];
+    shuffleArray(input);
+    expect(input).toEqual(snapshot);
+  });
+
+  it('preserves the multiset of elements (same length, same members)', () => {
+    const input = ['a', 'b', 'c', 'd', 'e'];
+    const out = shuffleArray(input);
+    expect(out).toHaveLength(input.length);
+    expect([...out].sort()).toEqual([...input].sort());
+  });
+
+  it('returns a copy (not the same reference)', () => {
+    const input = [1, 2, 3];
+    expect(shuffleArray(input)).not.toBe(input);
+  });
+
+  it('returns an empty array when called with an empty array', () => {
+    expect(shuffleArray([])).toEqual([]);
+  });
+
+  it('returns the single-element input unchanged', () => {
+    expect(shuffleArray(['only'])).toEqual(['only']);
+  });
+
+  it('produces a deterministic order under a mocked RNG (Math.random=0 picks j=0 each iteration)', () => {
+    vi.spyOn(Math, 'random').mockReturnValue(0);
+    // With Math.random()=0, j=floor(0 * (i+1))=0 for every i. The Fisher-Yates
+    // step swaps arr[i] with arr[0]. Walk it through for [1,2,3,4]:
+    //   i=3: swap(3,0) → [4,2,3,1]
+    //   i=2: swap(2,0) → [3,2,4,1]
+    //   i=1: swap(1,0) → [2,3,4,1]
+    expect(shuffleArray([1, 2, 3, 4])).toEqual([2, 3, 4, 1]);
+  });
+});