feat: migrate Leptos frontend to Next.js 16 (React 19)

Complete migration from services/frontend.old/ (Leptos 0.7 WASM + Rust)
to services/frontend/ (Next.js 16 static export + TypeScript + Tailwind v4).

Summary:
- Port all shared types (message, guild, voice, media, dashboard, recording, ui)
- Build fetch-based API client covering all 30+ backend endpoints
- WebSocket client with auto-reconnect (exponential backoff, 20 attempts)
- React context provider for WS with typed event subscription (22 event types)
- Login page with localStorage auth + auto-redirect
- Dashboard layout with sidebar, header (WS status + theme toggle)
- Messages: feed, search, images tab, review tab, channel filter, detail modal
- Live: voice connection, music player, recordings, mic transmit, active speakers
- Dashboard: stats, user list, channel list, detail views
- Mascot chatbot with history + clear
- uiStateApi persistence for selected tab
- Add static export config, update deploy scripts and CI

services/frontend/src/lib/api/mascot.ts

@@ -0,0 +1,11 @@
+import type { ChatHistoryMessage, MascotChatResponse } from "@/lib/types";
+import { api } from "./client";
+
+export const mascotApi = {
+  send: (message: string) =>
+    api.post<MascotChatResponse>("/api/mascot/chat", { message }),
+
+  getHistory: () => api.get<ChatHistoryMessage[]>("/api/mascot/chat/history"),
+
+  clearHistory: () => api.delete<{ ok: boolean }>("/api/mascot/chat/history"),
+};