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/routes/settings/+page.svelte

@@ -30,6 +30,7 @@
   import MobileAppDownload from '$lib/components/MobileAppDownload.svelte';
   import ServerVersion from '$lib/components/ServerVersion.svelte';
   import ActiveSessions from '$lib/components/ActiveSessions.svelte';
+  import NormalizationSetting from '$lib/components/NormalizationSetting.svelte';
 
   const queryClient = useQueryClient();
 
@@ -341,6 +342,8 @@
         0 = off · most albums sound best at 0.
       </p>
     </div>
+
+    <NormalizationSetting />
   </section>
 
   <section class="space-y-4 rounded border border-border bg-surface p-4">

web/src/routes/settings/Appearance.test.ts

@@ -25,6 +25,16 @@ vi.mock('$lib/api/listenbrainz', () => {
   };
 });
 
+// The Volume leveling setting loads on mount; keep it off the network.
+vi.mock('$lib/api/normalization', async (importOriginal) => {
+  const mod = await importOriginal<typeof import('$lib/api/normalization')>();
+  return {
+    ...mod,
+    getNormalization: vi.fn().mockResolvedValue(mod.DEFAULT_NORMALIZATION),
+    putNormalization: vi.fn()
+  };
+});
+
 vi.mock('$lib/api/metrics', () => ({
   createRecommendationMetricsQuery: () => ({
     subscribe: (run: (v: unknown) => void) => {

web/src/routes/settings/settings.test.ts

@@ -11,6 +11,16 @@ vi.mock('$lib/api/listenbrainz', () => ({
   setListenBrainzEnabled: vi.fn()
 }));
 
+// The Volume leveling setting loads on mount; keep it off the network.
+vi.mock('$lib/api/normalization', async (importOriginal) => {
+  const mod = await importOriginal<typeof import('$lib/api/normalization')>();
+  return {
+    ...mod,
+    getNormalization: vi.fn().mockResolvedValue(mod.DEFAULT_NORMALIZATION),
+    putNormalization: vi.fn()
+  };
+});
+
 vi.mock('$lib/api/me', () => ({
   updateProfile: vi.fn(),
   changePassword: vi.fn(),