feat: per-user volume leveling preference, synced across devices (M464 #4998)

Mode (off, auto, track, album), target (-18, -16, -14 LUFS) and boost
(within headroom, or fully with a limiter), stored per user on the server
so the web player, the Android app and casts apply the same one.

- Server: user_normalization_prefs (migration 0067), GET/PUT
  /api/me/normalization; a whole-body PUT, validated, last write wins.
- Web: Settings > Playback > Volume leveling. Saves at once, restores the
  old choice if the save fails, and caches the value for the player.
- Android: Settings card. The device keeps a copy for offline playback
  (Room v9 with an explicit migration, so the upgrade wipes nothing).
  Writes are offline-first: shown at once, PUT best effort, queued on
  failure (NORMALIZATION_SET, collapsed to the newest). A refresh never
  overwrites a change still queued.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/components/NormalizationSetting.test.ts

@@ -0,0 +1,65 @@
+import { afterEach, describe, expect, test, vi } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
+
+vi.mock('$lib/api/normalization', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('$lib/api/normalization')>()),
+  getNormalization: vi.fn(),
+  putNormalization: vi.fn()
+}));
+vi.mock('$lib/stores/toast.svelte', () => ({ pushToast: vi.fn() }));
+
+import NormalizationSetting from './NormalizationSetting.svelte';
+import { getNormalization, putNormalization } from '$lib/api/normalization';
+import { normalization } from '$lib/stores/normalization.svelte';
+import { pushToast } from '$lib/stores/toast.svelte';
+
+afterEach(() => {
+  vi.clearAllMocks();
+  localStorage.clear();
+});
+
+describe('NormalizationSetting', () => {
+  test('shows the server value and saves the whole preference on a change', async () => {
+    vi.mocked(getNormalization).mockResolvedValue({ mode: 'track', target_lufs: -16, boost: 'headroom' });
+    vi.mocked(putNormalization).mockImplementation(async (p) => p);
+    render(NormalizationSetting);
+
+    await waitFor(() => expect(screen.getByRole('radio', { name: 'Track' })).toBeChecked());
+    expect(screen.getByRole('radio', { name: '-16 LUFS' })).toBeChecked();
+
+    await fireEvent.click(screen.getByRole('radio', { name: 'Album' }));
+    await waitFor(() =>
+      expect(putNormalization).toHaveBeenCalledWith({ mode: 'album', target_lufs: -16, boost: 'headroom' })
+    );
+    await fireEvent.click(screen.getByRole('checkbox', { name: /boost quiet tracks/i }));
+    await waitFor(() =>
+      expect(putNormalization).toHaveBeenLastCalledWith({ mode: 'album', target_lufs: -16, boost: 'limiter' })
+    );
+    // The last value seen is cached for the player's first track of a session.
+    expect(JSON.parse(localStorage.getItem('minstrel:normalization') ?? 'null')).toEqual({
+      mode: 'album',
+      target_lufs: -16,
+      boost: 'limiter'
+    });
+  });
+
+  test('a failed save puts the previous choice back and says so', async () => {
+    vi.mocked(getNormalization).mockResolvedValue({ mode: 'auto', target_lufs: -18, boost: 'headroom' });
+    vi.mocked(putNormalization).mockRejectedValue(new Error('offline'));
+    render(NormalizationSetting);
+    await waitFor(() => expect(screen.getByRole('radio', { name: 'Auto' })).toBeChecked());
+
+    await fireEvent.click(screen.getByRole('radio', { name: 'Off' }));
+    await waitFor(() => expect(pushToast).toHaveBeenCalledWith('Could not save volume leveling.', 'error'));
+    expect(normalization.value.mode).toBe('auto');
+    expect(screen.getByRole('radio', { name: 'Auto' })).toBeChecked();
+  });
+
+  test('off hides the target and boost controls', async () => {
+    vi.mocked(getNormalization).mockResolvedValue({ mode: 'off', target_lufs: -18, boost: 'headroom' });
+    render(NormalizationSetting);
+    await waitFor(() => expect(screen.getByRole('radio', { name: 'Off' })).toBeChecked());
+    expect(screen.queryByRole('radio', { name: '-18 LUFS' })).toBeNull();
+    expect(screen.queryByRole('checkbox')).toBeNull();
+  });
+});