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]>

frontend/package.json

@@ -4,25 +4,45 @@
   "version": "1.0.0",
   "type": "module",
   "scripts": {
-    "dev": "vite --host 0.0.0.0 --port 5173",
-    "build": "tsc -b && vite build",
-    "preview": "vite preview --host 0.0.0.0"
+    "dev": "vite --host 0.0.0.0 --port 5175",
+    "build": "tsc --noEmit && vite build",
+    "preview": "vite preview --host 0.0.0.0 --port 5175"
   },
   "dependencies": {
-    "@ant-design/icons": "^5.4.0",
-    "antd": "^5.21.0",
+    "@radix-ui/react-alert-dialog": "^1.1.23",
+    "@radix-ui/react-dialog": "^1.1.23",
+    "@radix-ui/react-dropdown-menu": "^2.1.24",
+    "@radix-ui/react-label": "^2.1.15",
+    "@radix-ui/react-popover": "^1.1.23",
+    "@radix-ui/react-progress": "^1.1.16",
+    "@radix-ui/react-select": "^2.3.7",
+    "@radix-ui/react-separator": "^1.1.15",
+    "@radix-ui/react-slot": "^1.3.3",
+    "@radix-ui/react-switch": "^1.3.7",
+    "@radix-ui/react-tabs": "^1.1.21",
+    "@radix-ui/react-tooltip": "^1.2.16",
     "axios": "^1.7.7",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
     "hls.js": "^1.5.14",
+    "lucide-react": "^0.468.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-markdown": "^10.1.0",
     "react-router-dom": "^6.26.2",
-    "remark-gfm": "^4.0.1"
+    "remark-gfm": "^4.0.1",
+    "sonner": "^1.7.1",
+    "tailwind-merge": "^2.5.4"
   },
   "devDependencies": {
+    "@types/node": "^22.7.4",
     "@types/react": "^18.3.8",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
+    "autoprefixer": "^10.5.6",
+    "postcss": "^8.5.28",
+    "tailwindcss": "^3.4.17",
+    "tailwindcss-animate": "^1.0.7",
     "typescript": "^5.6.2",
     "vite": "^5.4.7"
   }