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/previewStore.ts

@@ -1,8 +1,9 @@
+import type { TrackPreviewLocation } from './authStoreTypes';
 import { create } from 'zustand';
 import { invoke } from '@tauri-apps/api/core';
 import { buildStreamUrl } from '../api/subsonic';
 import { usePlayerStore } from './playerStore';
-import { useAuthStore, type TrackPreviewLocation } from './authStore';
+import { useAuthStore } from './authStore';
 import { useOrbitStore } from './orbitStore';
 
 /** Minimal track info needed to surface the preview in the player bar UI. */