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/stores/normalization.svelte.ts

@@ -0,0 +1,66 @@
+import * as storage from '$lib/util/safeLocalStorage';
+import {
+  DEFAULT_NORMALIZATION,
+  getNormalization,
+  isNormalizationPrefs,
+  putNormalization,
+  type NormalizationPrefs
+} from '$lib/api/normalization';
+
+// The signed-in user's normalization preference. The server holds it; this
+// keeps the last value seen in localStorage too, so the player levels the
+// first track of a session before the fetch returns.
+const STORAGE_KEY = 'minstrel:normalization';
+
+function readCached(): NormalizationPrefs {
+  try {
+    const v: unknown = JSON.parse(storage.read(STORAGE_KEY) ?? 'null');
+    return isNormalizationPrefs(v) ? v : DEFAULT_NORMALIZATION;
+  } catch {
+    return DEFAULT_NORMALIZATION;
+  }
+}
+
+let _prefs = $state<NormalizationPrefs>(readCached());
+// Bumped by every change made here. A load or a save that resolves after a
+// newer change must not overwrite it.
+let _generation = 0;
+
+export const normalization = {
+  get value(): NormalizationPrefs {
+    return _prefs;
+  }
+};
+
+function commit(p: NormalizationPrefs): void {
+  _prefs = p;
+  storage.write(STORAGE_KEY, JSON.stringify(p));
+}
+
+/** Refreshes from the server; on failure the cached value stands. */
+export async function loadNormalization(): Promise<void> {
+  const gen = _generation;
+  try {
+    const p = await getNormalization();
+    if (gen === _generation) commit(p);
+  } catch {
+    // Offline or signed out: keep what we had.
+  }
+}
+
+/**
+ * Applies [next] at once, then saves it. If the save fails the previous
+ * value is restored and the error rethrown for the caller to report.
+ */
+export async function setNormalization(next: NormalizationPrefs): Promise<void> {
+  const prev = _prefs;
+  const gen = ++_generation;
+  commit(next);
+  try {
+    const saved = await putNormalization(next);
+    if (gen === _generation) commit(saved);
+  } catch (err) {
+    if (gen === _generation) commit(prev);
+    throw err;
+  }
+}