refactor(auth): E.45 — extract seven settings factories (#610)

Seven small domain-eng factories peel ~53 trivial setters out of the
authStore body:

- `createCacheStorageActions` — disk caches (max cache MB, download
  folder, offline download dir, hot-cache enable/MB/debounce/dir).
- `createDiscordSettingsActions` — rich presence, cover source,
  Bandsintown toggle, three template strings.
- `createUiAppearanceActions` — visual chrome: tray, titlebar, sidebar
  toggles, fullscreen-lyrics rendering, changelog banner, seekbar
  style, queue collapse, kinetic scroll, mini-player preload.
- `createLyricsSettingsActions` — lyrics fetch pipeline (server-first,
  netease enablement, source order, mode, static-only).
- `createTrackPreviewActions` — preview enable + per-location +
  start ratio (clamped 0…0.9) + duration (clamped 5…120s).
- `createDiscoveryActions` — mix min-rating filters (clamped 0…3),
  random-mix size (snapped to allowed bucket), lucky-mix visibility,
  random-nav mode, infinite-queue, preserve-play-next-order.
- `createPlumbingSettingsActions` — logging mode, getNowPlaying
  toggle, preload mode + custom seconds, audiobook exclusion,
  genre blacklist.

Pure code-move. authStore.ts: 428 → 384 LOC (−44).

src/store/authDiscoveryActions.ts

@@ -0,0 +1,38 @@
+import { clampMixFilterMinStars, clampRandomMixSize } from './authStoreHelpers';
+import type { AuthState } from './authStoreTypes';
+
+type SetState = (
+  partial: Partial<AuthState> | ((state: AuthState) => Partial<AuthState>),
+) => void;
+
+/**
+ * Discovery + auto-queue settings: mix min-rating thresholds (each
+ * clamped to 0…3 stars), random-mix size (snapped to the allowed
+ * RANDOM_MIX_SIZE_OPTIONS bucket), lucky-mix sidebar visibility,
+ * random-nav style, infinite-queue + preserve-play-next-order
+ * toggles.
+ */
+export function createDiscoveryActions(set: SetState): Pick<
+  AuthState,
+  | 'setMixMinRatingFilterEnabled'
+  | 'setMixMinRatingSong'
+  | 'setMixMinRatingAlbum'
+  | 'setMixMinRatingArtist'
+  | 'setRandomMixSize'
+  | 'setShowLuckyMixMenu'
+  | 'setRandomNavMode'
+  | 'setInfiniteQueueEnabled'
+  | 'setPreservePlayNextOrder'
+> {
+  return {
+    setMixMinRatingFilterEnabled: (v) => set({ mixMinRatingFilterEnabled: v }),
+    setMixMinRatingSong: (v) => set({ mixMinRatingSong: clampMixFilterMinStars(v) }),
+    setMixMinRatingAlbum: (v) => set({ mixMinRatingAlbum: clampMixFilterMinStars(v) }),
+    setMixMinRatingArtist: (v) => set({ mixMinRatingArtist: clampMixFilterMinStars(v) }),
+    setRandomMixSize: (v) => set({ randomMixSize: clampRandomMixSize(v) }),
+    setShowLuckyMixMenu: (v) => set({ showLuckyMixMenu: v }),
+    setRandomNavMode: (v) => set({ randomNavMode: v }),
+    setInfiniteQueueEnabled: (v) => set({ infiniteQueueEnabled: v }),
+    setPreservePlayNextOrder: (v) => set({ preservePlayNextOrder: v }),
+  };
+}