feat: consolidate on the shadcn frontend and harden playback

Frontend
- Replace the Ant Design app with the shadcn/ui app as the single frontend,
  keeping the portfolio side-by-side revertible in this commit.
- Drop the abandoned mdui experiment and the AntD tree so there is one app,
  one router and one design system to maintain.

Live playback
- Stop racing the encoder. lowLatencyMode + liveSyncDurationCount:1 targeted a
  single 4s segment, so playback ran at up to 1.10x and the forward buffer
  bottomed out at 0.1s, re-buffering every segment. Measured over 40s:
  buffer min 0.1s -> 8.9s, rate 1.00 constant, 0 stalls.
- Remove the 落后 labels and the page-level edge-snapping timer that fought
  hls.js over two different definitions of "latency".
- Live can no longer be paused: the control is not rendered while it plays, so
  the icon can never misreport state.
- Fix autoplay: muted autoplay started silently and never unmuted, and the
  gesture that restored sound also toggled playback off. One owner now.

Source ("原画") rendition
- Publish the publisher's stream remuxed losslessly via -c:v copy alongside the
  transcoded ones, so a source option exists even when every ladder entry
  downsizes.
- Give renditions an ASCII path slug: the HLS path is part of the URL and
  MediaMTX rejects non-alphanumeric paths, so 原画 could never be served.

Backend
- Supervise MediaMTX from the backend: start it at boot and restart it on exit,
  never killing an origin it did not start.
- Clear stale HLS output before restarting an encoder; an ENDLIST-less playlist
  left by a dead encoder hangs hls.js forever.
- Mark JSON timestamps as UTC. utcnow().isoformat() emitted a naive string that
  JS read as local time, so every viewer heartbeat looked 8 hours old in UTC+8
  and the live board's active-viewer count was permanently zero.
- Rebind a viewer session when it arrives on another stream instead of failing,
  and survive the StrictMode double-heartbeat race on first insert.
- Replace the Linux-only /proc and os.getloadavg metrics with psutil.
- Complete MediaMTX's cookieCheck redirect inside /hls proxying, and hold the
  httpx client open for the streamed body.

Housekeeping
- Ignore RainOSS (it stores accessKeySecret), runtime output, TLS keys and
  .venv; remove generated screenshots, load test scripts and stray files.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

backend/config.py

@@ -16,6 +16,15 @@ class Settings(BaseSettings):
     media_origin_rtmp_port: int = 1935
     media_origin_api_port: int = 9997
     media_origin_hls_port: int = 8888
+    # MediaMTX is supervised by the backend: started at boot if the origin is not
+    # already answering, and restarted if it dies. Set LS_MEDIA_ORIGIN_AUTOSTART=false
+    # to manage it externally (e.g. systemd, or an edge node's own launcher).
+    media_origin_autostart: bool = True
+    # "mediamtx" resolves via PATH; the repo-local copies are the fallback so a
+    # checkout works without a system-wide install. On Windows the binary sits in
+    # mediamtx\, on Linux/macOS it is usually a bare `mediamtx` on PATH.
+    media_origin_binary: str = "mediamtx"
+    media_origin_config: str = str(APP_DIR.parent / "mediamtx" / "mediamtx.yml")
     replay_media_dir: str = str(APP_DIR.parent / "media")
     hls_runtime_dir: str = str(APP_DIR.parent / "runtime" / "hls")
     hls_segment_seconds: int = 4
@@ -43,6 +52,17 @@ class Settings(BaseSettings):
         {"label": "360p", "width": 640, "height": 360, "bitrate": "600k"},
     ]
 
+    # Extra rendition that is published alongside the configured ones: the
+    # publisher's own video+audio stream remuxed into HLS without re-encoding.
+    # It costs almost no CPU (stream copy) and gives viewers a lossless
+    # "原画" (source) option even when every configured rendition downsizes.
+    source_rendition_label: str = "原画"
+    source_rendition_enabled: bool = True
+    # On-disk/URL name for that rendition. It IS the HLS path segment, and MediaMTX
+    # rejects any path containing non-alphanumeric characters, so the Chinese
+    # display label above cannot be used here — a CI would 500 trying to serve it.
+    source_rendition_slug: str = "source"
+
     transcode_profiles: dict[str, list[dict]] = {
         "standard": qualities,
         "bandwidth-saver": [