refactor(auth): E.47 — extract onRehydrateStorage migration block (#612)

The persist middleware's `onRehydrateStorage` callback was ~100 LOC
of legacy-shape migrations: hot-cache/preload mutual-exclusion reset,
lyricsServerFirst+enableNeteaselyrics → lyricsSources one-time
migration, Linux smooth-scroll one-shot, seekbar `'waveform'` →
`'truewave'` rename, animationMode/reducedAnimations cruft strip,
loudnessPreIsRefV1 reference recalibration, enableAppleMusicCoversDiscord
→ discordCoverSource enum migration, plus the sanitizers for
mixMinRating / randomMixSize / skipStarManualSkipCountsByKey.

Moved into `computeAuthStoreRehydration(state) → Partial<AuthState>`
in `authStoreRehydrate.ts`; the store's callback is now a four-line
wrapper. Pure code-move — same migration logic, same call order.

authStore.ts: 270 → 158 LOC (−112). Total Phase-E-auth journey:
889 → 158 LOC (−82%). All non-trivial content is now out of the
authStore body; what remains is state-init, 13 factory spreads, 2
derived getters, persist config, and the thin onRehydrate wrapper.

src/store/authStoreRehydrate.ts

@@ -0,0 +1,131 @@
+import { IS_LINUX } from '../utils/platform';
+import {
+  LOUDNESS_PRE_ANALYSIS_REF_TARGET_LUFS,
+  clampStoredLoudnessPreAnalysisAttenuationRefDb,
+} from '../utils/loudnessPreAnalysisSlider';
+import { DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB } from './authStoreDefaults';
+import {
+  clampMixFilterMinStars,
+  clampRandomMixSize,
+  sanitizeLoudnessLufsPreset,
+  sanitizeLoudnessPreAnalysisFromStorage,
+  sanitizeSkipStarCounts,
+} from './authStoreHelpers';
+import type {
+  AuthState,
+  DiscordCoverSource,
+  LyricsSourceConfig,
+  SeekbarStyle,
+} from './authStoreTypes';
+
+/**
+ * Computes the post-rehydration patch for the auth store. Runs all
+ * legacy-shape migrations + numeric sanitization that the persist
+ * middleware can't express declaratively. The caller (the store's
+ * `onRehydrateStorage` callback) applies the returned partial via
+ * `useAuthStore.setState`.
+ *
+ * Side effects this function takes: deletes obsolete legacy fields
+ * directly off the rehydrated state object (`animationMode`,
+ * `reducedAnimations`) so they don't sit as cruft in localStorage,
+ * and writes the one-shot Linux smooth-scroll migration sentinel.
+ */
+export function computeAuthStoreRehydration(state: AuthState): Partial<AuthState> {
+  // If both hot cache and preload were enabled before mutual exclusion was enforced, reset both.
+  const conflictingLegacyState =
+    state.hotCacheEnabled && state.preloadMode !== 'off'
+      ? { hotCacheEnabled: false, preloadMode: 'off' as const }
+      : {};
+
+  // Migrate lyricsServerFirst + enableNeteaselyrics → lyricsSources (one-time).
+  let lyricsSourcesMigrated: { lyricsSources?: LyricsSourceConfig[] } = {};
+  try {
+    const raw = JSON.parse(localStorage.getItem('psysonic-auth') ?? '{}') as { state?: Record<string, unknown> };
+    if (!raw?.state?.lyricsSources) {
+      const serverFirst = (raw?.state?.lyricsServerFirst as boolean | undefined) ?? true;
+      const neteaseOn   = (raw?.state?.enableNeteaselyrics as boolean | undefined) ?? false;
+      const migrated: LyricsSourceConfig[] = serverFirst
+        ? [{ id: 'server', enabled: true }, { id: 'lrclib', enabled: true }, { id: 'netease', enabled: neteaseOn }]
+        : [{ id: 'lrclib', enabled: true }, { id: 'server', enabled: true }, { id: 'netease', enabled: neteaseOn }];
+      lyricsSourcesMigrated = { lyricsSources: migrated };
+    }
+  } catch { /* ignore */ }
+
+  // One-time: older builds could persist smooth=false as the default. Force smooth on once
+  // so updates do not leave users on discrete scrolling; after this flag exists, only an
+  // explicit toggle in Settings may turn it off (persisted in psysonic-auth).
+  const wheelSmoothMigrationKey = 'psysonic-linux-webkit-smooth-v1';
+  let wheelSmoothOneTime: { linuxWebkitKineticScroll?: boolean } = {};
+  if (IS_LINUX) {
+    try {
+      if (!localStorage.getItem(wheelSmoothMigrationKey)) {
+        wheelSmoothOneTime = { linuxWebkitKineticScroll: true };
+        localStorage.setItem(wheelSmoothMigrationKey, '1');
+      }
+    } catch { /* ignore */ }
+  }
+
+  // 'waveform' style was renamed to 'truewave' (with 'pseudowave' added
+  // as the deterministic legacy variant). Any persisted value that is
+  // not a valid SeekbarStyle (legacy 'waveform', undefined, tampered
+  // strings) lands on the new bins-based default — otherwise the
+  // dispatcher's switch finds no match and the seekbar renders blank.
+  const VALID_SEEKBAR_STYLES = new Set<string>([
+    'truewave', 'pseudowave', 'linedot', 'bar', 'thick',
+    'segmented', 'neon', 'pulsewave', 'particletrail', 'liquidfill', 'retrotape',
+  ]);
+  const seekbarStyleMigrated = VALID_SEEKBAR_STYLES.has(state.seekbarStyle as string)
+    ? {}
+    : { seekbarStyle: 'truewave' as SeekbarStyle };
+
+  // The `animationMode` 3-state setting was removed; users on `'reduced'`
+  // or `'static'` collapse onto the former `'full'` path automatically as
+  // soon as the field is gone from the store. Strip the persisted field
+  // so it doesn't sit in localStorage as cruft.
+  delete (state as { animationMode?: unknown }).animationMode;
+  // The earlier `reducedAnimations: boolean` predecessor likewise loses
+  // its meaning; clear it for the same reason.
+  delete (state as { reducedAnimations?: unknown }).reducedAnimations;
+
+  const st = state as {
+    loudnessTargetLufs?: unknown;
+    loudnessPreAnalysisAttenuationDb?: unknown;
+    loudnessPreIsRefV1?: unknown;
+  };
+  const targetSan = sanitizeLoudnessLufsPreset(st.loudnessTargetLufs, -12);
+  const rawN = st.loudnessPreAnalysisAttenuationDb;
+  const n = typeof rawN === 'number' ? rawN : Number(rawN);
+  const preSan =
+    st.loudnessPreIsRefV1 === true
+      ? sanitizeLoudnessPreAnalysisFromStorage(rawN)
+      : (Number.isFinite(n)
+          ? clampStoredLoudnessPreAnalysisAttenuationRefDb(
+              n - (targetSan - LOUDNESS_PRE_ANALYSIS_REF_TARGET_LUFS),
+            )
+          : DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB);
+
+  // Migrate enableAppleMusicCoversDiscord boolean → discordCoverSource enum.
+  let discordCoverSourceMigrated: { discordCoverSource?: DiscordCoverSource } = {};
+  const legacyAppleCovers = (state as { enableAppleMusicCoversDiscord?: unknown }).enableAppleMusicCoversDiscord;
+  if (legacyAppleCovers === true && (!state.discordCoverSource || state.discordCoverSource === 'none')) {
+    discordCoverSourceMigrated = { discordCoverSource: 'apple' };
+  }
+
+  return {
+    mixMinRatingSong: clampMixFilterMinStars(state.mixMinRatingSong as number),
+    mixMinRatingAlbum: clampMixFilterMinStars(state.mixMinRatingAlbum as number),
+    mixMinRatingArtist: clampMixFilterMinStars(state.mixMinRatingArtist as number),
+    randomMixSize: clampRandomMixSize(state.randomMixSize as number),
+    skipStarManualSkipCountsByKey: sanitizeSkipStarCounts(
+      (state as { skipStarManualSkipCountsByKey?: unknown }).skipStarManualSkipCountsByKey,
+    ),
+    loudnessTargetLufs: targetSan,
+    loudnessPreAnalysisAttenuationDb: preSan,
+    loudnessPreIsRefV1: true,
+    ...conflictingLegacyState,
+    ...lyricsSourcesMigrated,
+    ...wheelSmoothOneTime,
+    ...seekbarStyleMigrated,
+    ...discordCoverSourceMigrated,
+  };
+}