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/tabs.tsx

@@ -0,0 +1,35 @@
+import * as TabsPrimitive from "@radix-ui/react-tabs";
+import type * as React from "react";
+import { cn } from "../../lib/utils";
+
+export const Tabs = TabsPrimitive.Root;
+
+export function TabsList({ className, ...props }: React.ComponentPropsWithoutRef<typeof TabsPrimitive.List>) {
+  return (
+    <TabsPrimitive.List
+      className={cn("inline-flex h-10 items-center justify-center rounded-lg bg-muted p-1 text-muted-foreground", className)}
+      {...props}
+    />
+  );
+}
+
+export function TabsTrigger({ className, ...props }: React.ComponentPropsWithoutRef<typeof TabsPrimitive.Trigger>) {
+  return (
+    <TabsPrimitive.Trigger
+      className={cn(
+        "inline-flex items-center justify-center whitespace-nowrap rounded-md px-3 py-1.5 text-sm font-medium transition-all focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring disabled:pointer-events-none disabled:opacity-50 data-[state=active]:bg-background data-[state=active]:text-foreground data-[state=active]:shadow-sm",
+        className,
+      )}
+      {...props}
+    />
+  );
+}
+
+export function TabsContent({ className, ...props }: React.ComponentPropsWithoutRef<typeof TabsPrimitive.Content>) {
+  return (
+    <TabsPrimitive.Content
+      className={cn("mt-6 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring", className)}
+      {...props}
+    />
+  );
+}