feat: make Jellyfin an optional source instead of the default

Revert the jellyfin-only default introduced by PR #123 so upgrading
users keep their online sources; Jellyfin becomes opt-in:

- default enabledProviders is now the online set (netease/qq/bilibili/
  youtube/kugou); defaultPlatform() uses a fixed priority order
  (netease -> qq -> kugou -> jellyfin -> bilibili -> youtube) instead
  of jellyfin-first, so chat/REST/WebUI default to netease again
- Settings: Jellyfin card is always visible with a new enable toggle
  (its enabled bit is enabledProviders membership); guards against
  clobbering other providers before the list loads
- Setup wizard: saving the Jellyfin step auto-enables the source when
  a server URL was entered
- Search/player store fallbacks flip from jellyfin to netease; !help
  no longer hardcodes Jellyfin lines
- tests: update default-platform assertions, add coverage for the new
  default set, legacy configs without enabledProviders, priority
  order, and explicit jellyfin-only configs
- README: reframe Jellyfin as optional (badges, command table, quality
  tiers, dedicated section, changelog), document the enabledProviders
  default and the v1.10.0 jellyfin-only window fix, credit @ItsEricRao

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/src/views/Search.vue

@@ -57,7 +57,7 @@
 
     <template v-else-if="allSongs.length || allAlbums.length || allPlaylists.length">
       <!-- Only enabled sources are offered (enabledProviders gate); Jellyfin
-           — the primary source — comes first. -->
+           (opt-in) comes first when enabled. -->
       <div class="source-bar">
         <button
           v-if="sourceEnabled('jellyfin')"
@@ -218,7 +218,7 @@ function loadSource(): SearchSource {
     const stored = localStorage.getItem(SOURCE_STORAGE_KEY);
     if (SEARCH_SOURCES.includes(stored as SearchSource)) return stored as SearchSource;
   } catch { /* localStorage blocked */ }
-  return 'jellyfin';
+  return 'netease';
 }
 
 type TabType = 'songs' | 'albums' | 'playlists';
@@ -272,7 +272,7 @@ function fixupSelectedSource() {
   const def = store.defaultSource as SearchSource;
   selectedSource.value = SEARCH_SOURCES.includes(def) && sourceEnabled(def)
     ? def
-    : SEARCH_SOURCES.find((s) => s !== 'local' && sourceEnabled(s)) ?? 'jellyfin';
+    : SEARCH_SOURCES.find((s) => s !== 'local' && sourceEnabled(s)) ?? 'netease';
 }
 
 // ---- 分页 / 加载更多 ----