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/authStoreHelpers.ts

@@ -0,0 +1,61 @@
+import { clampStoredLoudnessPreAnalysisAttenuationRefDb } from '../utils/loudnessPreAnalysisSlider';
+import {
+  DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB,
+  LOUDNESS_LUFS_PRESETS,
+  MIX_MIN_RATING_FILTER_MAX_STARS,
+  RANDOM_MIX_SIZE_OPTIONS,
+} from './authStoreDefaults';
+import type { LoudnessLufsPreset } from './authStoreTypes';
+
+export function generateId(): string {
+  return Date.now().toString(36) + Math.random().toString(36).slice(2);
+}
+
+export function sanitizeLoudnessLufsPreset(v: unknown, fallback: LoudnessLufsPreset): LoudnessLufsPreset {
+  return (LOUDNESS_LUFS_PRESETS as readonly number[]).includes(v as number)
+    ? (v as LoudnessLufsPreset)
+    : fallback;
+}
+
+export function sanitizeLoudnessPreAnalysisFromStorage(v: unknown): number {
+  const n = typeof v === 'number' ? v : Number(v);
+  if (!Number.isFinite(n)) return DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB;
+  return clampStoredLoudnessPreAnalysisAttenuationRefDb(n);
+}
+
+export function clampMixFilterMinStars(v: unknown): number {
+  const n = typeof v === 'number' ? v : Number(v);
+  if (!Number.isFinite(n)) return 0;
+  return Math.max(0, Math.min(MIX_MIN_RATING_FILTER_MAX_STARS, Math.round(n)));
+}
+
+export function clampRandomMixSize(v: number): number {
+  if (!Number.isFinite(v)) return 50;
+  // Snap to the nearest allowed option so a tampered persisted value can't break the picker.
+  let nearest = RANDOM_MIX_SIZE_OPTIONS[0];
+  let bestDelta = Math.abs(v - nearest);
+  for (const opt of RANDOM_MIX_SIZE_OPTIONS) {
+    const d = Math.abs(v - opt);
+    if (d < bestDelta) { nearest = opt; bestDelta = d; }
+  }
+  return nearest;
+}
+
+export function clampSkipStarThreshold(v: number): number {
+  if (!Number.isFinite(v)) return 3;
+  return Math.max(1, Math.min(99, Math.round(v)));
+}
+
+export function skipStarCountStorageKey(serverId: string | null | undefined, trackId: string): string {
+  return `${serverId ?? ''}\u001f${trackId}`;
+}
+
+export function sanitizeSkipStarCounts(raw: unknown): Record<string, number> {
+  if (!raw || typeof raw !== 'object' || Array.isArray(raw)) return {};
+  const next: Record<string, number> = {};
+  for (const [k, v] of Object.entries(raw as Record<string, unknown>)) {
+    const n = Number(v);
+    if (Number.isFinite(n) && n > 0) next[k] = Math.min(Math.floor(n), 1_000_000);
+  }
+  return next;
+}