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/ui/scroll-area.tsx

@@ -0,0 +1,30 @@
+import * as ScrollAreaPrimitive from "@radix-ui/react-scroll-area";
+import type * as React from "react";
+import { cn } from "../../lib/utils";
+
+export function ScrollArea({ className, children, ...props }: React.ComponentPropsWithoutRef<typeof ScrollAreaPrimitive.Root>) {
+  return (
+    <ScrollAreaPrimitive.Root className={cn("relative overflow-hidden", className)} {...props}>
+      <ScrollAreaPrimitive.Viewport className="h-full w-full rounded-[inherit]">{children}</ScrollAreaPrimitive.Viewport>
+      <ScrollBar />
+      <ScrollAreaPrimitive.Corner />
+    </ScrollAreaPrimitive.Root>
+  );
+}
+
+function ScrollBar({ className, orientation = "vertical", ...props }: React.ComponentPropsWithoutRef<typeof ScrollAreaPrimitive.ScrollAreaScrollbar>) {
+  return (
+    <ScrollAreaPrimitive.ScrollAreaScrollbar
+      orientation={orientation}
+      className={cn(
+        "flex touch-none select-none transition-colors",
+        orientation === "vertical" && "h-full w-2.5 border-l border-l-transparent p-[1px]",
+        orientation === "horizontal" && "h-2.5 flex-col border-t border-t-transparent p-[1px]",
+        className,
+      )}
+      {...props}
+    >
+      <ScrollAreaPrimitive.ScrollAreaThumb className="relative flex-1 rounded-full bg-border" />
+    </ScrollAreaPrimitive.ScrollAreaScrollbar>
+  );
+}