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>

src/index.ts

@@ -56,8 +56,8 @@ async function main() {
     {
       neteasePort: config.neteaseApiPort,
       qqMusicPort: config.qqMusicApiPort,
-      // Provider gating: with the default (jellyfin-only) config, neither
-      // sidecar starts and ports 3001/3200 are never bound.
+      // Provider gating: a sidecar only starts (and binds 3001/3200) when its
+      // source is listed in enabledProviders — both are on in the default config.
       neteaseEnabled: isProviderEnabled(config, "netease"),
       qqEnabled: isProviderEnabled(config, "qq"),
     },