refactor(auth): E.43 — extract authStore types + defaults + helpers (#608)

First slice of the authStore split. Pure code-move: no behaviour change.

- `authStoreTypes.ts` — `ServerProfile`, `AuthState`, plus all union
  types (`SeekbarStyle`, `LoggingMode`, `NormalizationEngine`,
  `DiscordCoverSource`, `LoudnessLufsPreset`, `LyricsSourceId`,
  `LyricsSourceConfig`, `TrackPreviewLocation`, `TrackPreviewLocations`).
- `authStoreDefaults.ts` — `LOUDNESS_LUFS_PRESETS`,
  `DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB`,
  `TRACK_PREVIEW_LOCATIONS`, `DEFAULT_TRACK_PREVIEW_LOCATIONS`,
  `DEFAULT_LYRICS_SOURCES`, `MIX_MIN_RATING_FILTER_MAX_STARS`,
  `RANDOM_MIX_SIZE_OPTIONS`.
- `authStoreHelpers.ts` — `generateId`,
  `sanitizeLoudnessLufsPreset`, `sanitizeLoudnessPreAnalysisFromStorage`,
  `clampMixFilterMinStars`, `clampRandomMixSize`,
  `clampSkipStarThreshold`, `skipStarCountStorageKey`,
  `sanitizeSkipStarCounts`.

12 external call sites migrated to direct imports from the new
modules (no re-export shims left in authStore.ts — applies the
[feedback_prevent_god_modules] rule 5: avoid re-export debt).

authStore.ts: 889 → 518 LOC (−371).

src/store/authStoreDefaults.ts

@@ -0,0 +1,40 @@
+import type {
+  LoudnessLufsPreset,
+  LyricsSourceConfig,
+  TrackPreviewLocation,
+  TrackPreviewLocations,
+} from './authStoreTypes';
+
+export const LOUDNESS_LUFS_PRESETS: LoudnessLufsPreset[] = [-16, -14, -12, -10];
+
+/** Settings default + Rust engine cold default until `audio_set_normalization` runs. */
+export const DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB = -4.5;
+
+export const TRACK_PREVIEW_LOCATIONS: readonly TrackPreviewLocation[] = [
+  'suggestions',
+  'albums',
+  'playlists',
+  'favorites',
+  'artist',
+  'randomMix',
+];
+
+export const DEFAULT_TRACK_PREVIEW_LOCATIONS: TrackPreviewLocations = {
+  suggestions: true,
+  albums: true,
+  playlists: true,
+  favorites: true,
+  artist: true,
+  randomMix: true,
+};
+
+export const DEFAULT_LYRICS_SOURCES: LyricsSourceConfig[] = [
+  { id: 'server',  enabled: true  },
+  { id: 'lrclib',  enabled: true  },
+  { id: 'netease', enabled: false },
+];
+
+/** Upper bound for mix min-rating thresholds (UI shows five stars, only 1…this many are selectable). */
+export const MIX_MIN_RATING_FILTER_MAX_STARS = 3;
+
+export const RANDOM_MIX_SIZE_OPTIONS: readonly number[] = [50, 75, 100, 125, 150];