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/layout/Sidebar.tsx

@@ -0,0 +1,48 @@
+import { Bot, MessageSquare, Music2, ShieldAlert, Volume2 } from "lucide-react";
+import type { DashboardTab } from "../../types/ui";
+import { cn } from "../../lib/utils";
+import { Button } from "../ui/button";
+
+const navItems: Array<{ id: DashboardTab; label: string; icon: typeof Volume2 }> = [
+  { id: "voice", label: "Voice", icon: Volume2 },
+  { id: "media", label: "Media", icon: Music2 },
+  { id: "messages", label: "Messages", icon: MessageSquare },
+  { id: "review", label: "Review", icon: ShieldAlert },
+];
+
+interface SidebarProps {
+  activeTab: DashboardTab;
+  onTabChange: (tab: DashboardTab) => void;
+}
+
+export function Sidebar({ activeTab, onTabChange }: SidebarProps) {
+  return (
+    <aside className="hidden w-72 shrink-0 border-r border-border bg-card/60 p-5 backdrop-blur md:block">
+      <div className="mb-8 flex items-center gap-3">
+        <div className="flex h-11 w-11 items-center justify-center rounded-2xl bg-primary/15 text-primary">
+          <Bot className="h-6 w-6" />
+        </div>
+        <div>
+          <div className="font-semibold tracking-tight">Bete Watcher</div>
+          <div className="text-xs text-muted-foreground">Discord control center</div>
+        </div>
+      </div>
+      <nav className="space-y-2">
+        {navItems.map((item) => {
+          const Icon = item.icon;
+          return (
+            <Button
+              key={item.id}
+              variant={activeTab === item.id ? "secondary" : "ghost"}
+              className={cn("w-full justify-start", activeTab === item.id && "bg-primary/15 text-primary")}
+              onClick={() => onTabChange(item.id)}
+            >
+              <Icon className="h-4 w-4" />
+              {item.label}
+            </Button>
+          );
+        })}
+      </nav>
+    </aside>
+  );
+}