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/authDiscordSettingsActions.ts

@@ -0,0 +1,24 @@
+import type { AuthState } from './authStoreTypes';
+
+type SetState = (
+  partial: Partial<AuthState> | ((state: AuthState) => Partial<AuthState>),
+) => void;
+
+export function createDiscordSettingsActions(set: SetState): Pick<
+  AuthState,
+  | 'setDiscordRichPresence'
+  | 'setDiscordCoverSource'
+  | 'setEnableBandsintown'
+  | 'setDiscordTemplateDetails'
+  | 'setDiscordTemplateState'
+  | 'setDiscordTemplateLargeText'
+> {
+  return {
+    setDiscordRichPresence: (v) => set({ discordRichPresence: v }),
+    setDiscordCoverSource: (v) => set({ discordCoverSource: v }),
+    setEnableBandsintown: (v) => set({ enableBandsintown: v }),
+    setDiscordTemplateDetails: (v) => set({ discordTemplateDetails: v }),
+    setDiscordTemplateState: (v) => set({ discordTemplateState: v }),
+    setDiscordTemplateLargeText: (v) => set({ discordTemplateLargeText: v }),
+  };
+}