feat(playback): client direct-play decode mode (release host transcoding)

Add an admin toggle playback.direct_only that offloads all decoding to the
client. When enabled:
- Emby PlaybackInfo no longer advertises SupportsTranscoding nor a
  TranscodingUrl, forcing Emby/Infuse/Yamby clients to direct play.
- HLS endpoints refuse (ErrTranscodeDisabled) so the host never spawns ffmpeg.
- Web player forces direct play, hides the HLS toggle, and surfaces a hint.
Exposed via /system/info (direct_play_only) and the Settings page.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

web/src/pages/SettingsPage.tsx

@@ -47,11 +47,18 @@ const GROUPS: SettingGroup[] = [
           { value: 'ja-JP', label: '日本語' },
         ],
       },
+      {
+        key: 'playback.direct_only',
+        label: '客户端直连解码（释放宿主机资源）',
+        type: 'toggle',
+        hint: '默认关闭。开启后宿主机不再进行任何 FFmpeg 转码，所有播放交给第三方客户端（Infuse / VLC / Emby 客户端等）或浏览器本地解码直连（direct play / 302 直链），大幅降低宿主机 CPU 占用。若客户端不支持源编码可能无法播放。',
+        defaultValue: 'false',
+      },
       {
         key: 'transcode.enabled',
         label: '启用转码',
         type: 'toggle',
-        hint: '关闭后所有视频直连播放',
+        hint: '关闭后所有视频直连播放（「客户端直连解码」开启时本项自动失效）',
         defaultValue: 'true',
       },
       {