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/components/ContextMenu.test.tsx

@@ -6,6 +6,7 @@
  * snapshots — tests survive a refactor that re-orders or re-styles the
  * markup as long as the menu items + their handlers stay observable.
  */
+import type { ServerProfile } from '@/store/authStoreTypes';
 import type { Track } from '@/store/playerStoreTypes';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
@@ -45,7 +46,7 @@ vi.mock('@/utils/orbitBulkGuard', () => ({
 import ContextMenu from './ContextMenu';
 import { renderWithProviders } from '@/test/helpers/renderWithProviders';
 import { usePlayerStore } from '@/store/playerStore';
-import { useAuthStore, type ServerProfile } from '@/store/authStore';
+import { useAuthStore } from '@/store/authStore';
 import { resetAllStores } from '@/test/helpers/storeReset';
 import { makeTrack, makeServer } from '@/test/helpers/factories';
 import { onInvoke } from '@/test/mocks/tauri';