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/src/index.css

@@ -0,0 +1,126 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+@layer base {
+  :root {
+    --background: 45 8% 11%;
+    --foreground: 42 24% 93%;
+    --card: 45 7% 14%;
+    --card-foreground: 42 24% 93%;
+    --popover: 45 7% 15%;
+    --popover-foreground: 42 24% 93%;
+    --primary: 43 90% 62%;
+    --primary-foreground: 40 30% 12%;
+    --secondary: 45 7% 18%;
+    --secondary-foreground: 42 24% 93%;
+    --muted: 45 7% 17%;
+    --muted-foreground: 43 12% 62%;
+    --accent: 45 7% 19%;
+    --accent-foreground: 42 24% 93%;
+    --destructive: 16 72% 60%;
+    --destructive-foreground: 42 24% 93%;
+    --border: 45 8% 21%;
+    --input: 45 8% 22%;
+    --ring: 43 90% 62%;
+    --radius: 0.625rem;
+  }
+}
+
+@layer base {
+  * {
+    @apply border-border;
+  }
+  body {
+    @apply bg-background text-foreground antialiased;
+    font-feature-settings: 'rlig' 1, 'calt' 1;
+    font-family: ui-sans-serif, -apple-system, BlinkMacSystemFont, 'Segoe UI', 'PingFang SC', 'Microsoft YaHei', sans-serif;
+  }
+}
+
+@layer utilities {
+  .font-display {
+    font-family: Georgia, 'Times New Roman', ui-serif, serif;
+  }
+  .shadow-warm {
+    box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.04), 0 12px 32px rgba(0, 0, 0, 0.22);
+  }
+  .shadow-warm-lg {
+    box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.05), 0 22px 58px rgba(0, 0, 0, 0.28);
+  }
+  .scrollbar-thin {
+    scrollbar-width: thin;
+    scrollbar-color: hsl(var(--border)) transparent;
+  }
+  .scrollbar-thin::-webkit-scrollbar {
+    width: 8px;
+    height: 8px;
+  }
+  .scrollbar-thin::-webkit-scrollbar-thumb {
+    background-color: hsl(var(--border));
+    border-radius: 8px;
+  }
+}
+
+/* Markdown content rendered by the viewer (description / chapters intro). */
+.md-content {
+  font-size: 0.875rem;
+  line-height: 1.7;
+  color: hsl(var(--foreground));
+}
+.md-content h1,
+.md-content h2,
+.md-content h3,
+.md-content h4 {
+  font-weight: 600;
+  margin: 1rem 0 0.5rem;
+}
+.md-content h1 {
+  font-size: 1.25rem;
+}
+.md-content h2 {
+  font-size: 1.125rem;
+}
+.md-content h3 {
+  font-size: 1rem;
+}
+.md-content p {
+  margin: 0.5rem 0;
+}
+.md-content ul,
+.md-content ol {
+  padding-left: 1.25rem;
+  margin: 0.5rem 0;
+}
+.md-content ul {
+  list-style: disc;
+}
+.md-content ol {
+  list-style: decimal;
+}
+.md-content a {
+  color: hsl(var(--primary));
+  text-decoration: underline;
+}
+.md-content code {
+  background: hsl(var(--muted));
+  padding: 0.125rem 0.375rem;
+  border-radius: 0.25rem;
+  font-size: 0.8rem;
+}
+.md-content pre {
+  background: hsl(var(--muted));
+  padding: 0.75rem;
+  border-radius: 0.375rem;
+  overflow-x: auto;
+}
+.md-content blockquote {
+  border-left: 3px solid hsl(var(--border));
+  padding-left: 0.75rem;
+  color: hsl(var(--muted-foreground));
+}
+
+/* Swagger UI sits inside a dark card; keep its own theme independent. */
+#swagger-container .swagger-ui {
+  font-family: inherit;
+}