feat: migrate and redesign dashboard to modern React

- Full rewrite of legacy vanilla JS UI into React SPA
- Implement modern design system using Tailwind CSS and shadcn/ui primitives
- Create typed API modules and hooks for voice, media, and moderation
- Add new features: separated Music and Screen Share panels, Image Grid
- Implement unified WebSocket hook for real-time state and PCM audio
- Improve visualizer with smooth CSS transitions and live state sync
- Add __dirname polyfill for ES module compatibility
- Ensure responsive layout for mobile and desktop

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/media/ScreenShare.tsx

@@ -0,0 +1,45 @@
+import { MonitorUp } from "lucide-react";
+import { useState } from "react";
+import { Button } from "../ui/button";
+import { Card, CardContent, CardDescription, CardHeader, CardTitle } from "../ui/card";
+import { Input } from "../ui/input";
+
+interface ScreenShareProps {
+  loading: boolean;
+  onStart: (source: string) => void;
+  onSkip: () => void;
+  onStop: () => void;
+}
+
+export function ScreenShare({ loading, onStart, onSkip, onStop }: ScreenShareProps) {
+  const [source, setSource] = useState("");
+
+  const submit = () => {
+    const trimmed = source.trim();
+    if (!trimmed) return;
+    onStart(trimmed);
+    setSource("");
+  };
+
+  return (
+    <Card>
+      <CardHeader>
+        <CardTitle className="flex items-center gap-2"><MonitorUp className="h-5 w-5" /> Screen Share</CardTitle>
+        <CardDescription>Start screen-share playback from a URL or local file path.</CardDescription>
+      </CardHeader>
+      <CardContent className="space-y-4">
+        <Input
+          value={source}
+          onChange={(event) => setSource(event.target.value)}
+          onKeyDown={(event) => event.key === "Enter" && submit()}
+          placeholder="Screen share URL or local file path"
+        />
+        <div className="flex flex-wrap gap-2">
+          <Button disabled={loading || !source.trim()} onClick={submit}>Start Screen Share</Button>
+          <Button variant="secondary" disabled={loading} onClick={onSkip}>Skip</Button>
+          <Button variant="destructive" disabled={loading} onClick={onStop}>Stop</Button>
+        </div>
+      </CardContent>
+    </Card>
+  );
+}