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/components/ui/card.tsx

@@ -0,0 +1,76 @@
+import * as React from "react"
+
+import { cn } from "@/lib/utils"
+
+const Card = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn(
+      "rounded-xl border border-border/60 bg-card text-card-foreground shadow-warm",
+      className
+    )}
+    {...props}
+  />
+))
+Card.displayName = "Card"
+
+const CardHeader = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("flex flex-col space-y-1.5 p-6", className)}
+    {...props}
+  />
+))
+CardHeader.displayName = "CardHeader"
+
+const CardTitle = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("font-semibold leading-none tracking-tight", className)}
+    {...props}
+  />
+))
+CardTitle.displayName = "CardTitle"
+
+const CardDescription = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("text-sm text-muted-foreground", className)}
+    {...props}
+  />
+))
+CardDescription.displayName = "CardDescription"
+
+const CardContent = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div ref={ref} className={cn("p-6 pt-0", className)} {...props} />
+))
+CardContent.displayName = "CardContent"
+
+const CardFooter = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("flex items-center p-6 pt-0", className)}
+    {...props}
+  />
+))
+CardFooter.displayName = "CardFooter"
+
+export { Card, CardHeader, CardFooter, CardTitle, CardDescription, CardContent }