refactor(playback): co-locate floating-player-bar + fs-idle-fade hooks (and orphaned playback tests) with their features

src/features/playback/hooks/computeFloatingPlayerBarStyle.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest';
+import { computeFloatingPlayerBarStyle } from '@/features/playback/hooks/computeFloatingPlayerBarStyle';
+
+describe('computeFloatingPlayerBarStyle', () => {
+  it('centers in the main column and shrink-wraps instead of stretching', () => {
+    const style = computeFloatingPlayerBarStyle(220, 1600, 1920);
+    expect(style.left).toBe(910);
+    expect(style.right).toBe('auto');
+    expect(style.transform).toBe('translateX(-50%)');
+    expect(style.width).toBe('max-content');
+    expect(style.maxWidth).toBe(1332);
+  });
+
+  it('accounts for a hidden queue panel', () => {
+    const style = computeFloatingPlayerBarStyle(220, null, 1920);
+    expect(style.left).toBe(1070);
+    expect(style.maxWidth).toBe(1652);
+  });
+});