feat(kugou): add login-gated discovery (daily/recommend/user playlists, FM) + covers

Implements the NetEase-parity login features for the Kugou provider now that
QR login works:
- getDailyRecommendSongs (每日推荐), getUserPlaylists (我的歌单), and a real
  getRecommendPlaylists (推荐歌单, was a stub) ported from the reference API.
- mapKugouSong now extracts cover art per endpoint (sizable_cover / cover /
  trans_param.union_cover, resolving the {size} template) — Kugou songs had no
  artwork before.
- Fix the playlist-song shape (combined "歌手 - 歌名" in `name`, mixsongid as the
  audio id) so opened playlists show real titles instead of 未知歌曲.
- New defensive playlist mappers keyed on global_collection_id (the only id
  getPlaylistSongs can open); dedup user playlists in case the list endpoint
  ignores pagination; firstStr() so an empty-string field can't mask a real one.

Frontend wires Kugou as a third home-discovery source (Source type, store
caches/auth, availableSources, fetchHomeData, Home FM card + source tabs,
SourceTabs label, persisted-tab whitelist). SourceTabs now highlights the
fallback-corrected source so the active tab shows when a logged-out source was
persisted (newly possible with 3 sources).

Adds kugou.test.ts coverage for the new mappers, the cover/empty-string and
playlist-shape handling, and id openability.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/views/Library.vue

@@ -28,7 +28,7 @@
       <h2 class="section-title">
         我的歌单
         <span v-if="currentUserPlaylists.length > 0" class="section-count">{{ currentUserPlaylists.length }}</span>
-        <SourceTabs v-model="userSource" :sources="userAvailable" />
+        <SourceTabs :model-value="userSourceSafe" @update:model-value="userSource = $event" :sources="userAvailable" />
       </h2>
       <div class="playlist-grid">
         <RouterLink