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/select.tsx

@@ -0,0 +1,31 @@
+import type * as React from "react";
+import { cn } from "../../lib/utils";
+
+export interface SelectOption {
+  value: string;
+  label: string;
+}
+
+interface SelectProps extends React.SelectHTMLAttributes<HTMLSelectElement> {
+  options: SelectOption[];
+  placeholder?: string;
+}
+
+export function Select({ className, options, placeholder, ...props }: SelectProps) {
+  return (
+    <select
+      className={cn(
+        "flex h-10 w-full rounded-lg border border-input bg-background px-3 py-2 text-sm text-foreground ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring disabled:cursor-not-allowed disabled:opacity-50",
+        className,
+      )}
+      {...props}
+    >
+      {placeholder ? <option value="">{placeholder}</option> : null}
+      {options.map((option) => (
+        <option key={option.value} value={option.value}>
+          {option.label}
+        </option>
+      ))}
+    </select>
+  );
+}