Initial commit: webmail client (M0/M1)

Three-pane webmail client.
- Backend: FastAPI + async SQLAlchemy + arq worker (Postgres/Redis), IMAP/SMTP
  sync, threading, search, drafts/outbox with undo, attachments, 2FA.
- Frontend: React 18 + Vite + TypeScript + TanStack Query + Tailwind; three-pane
  UI with rich-text composer, light/dark themes, resizable panels.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -0,0 +1,27 @@
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import React from "react";
+import ReactDOM from "react-dom/client";
+import { BrowserRouter } from "react-router-dom";
+
+import { App } from "./App";
+import { I18nProvider } from "./i18n";
+import { ThemeProvider } from "./lib/theme";
+import "./index.css";
+
+const queryClient = new QueryClient({
+  defaultOptions: { queries: { retry: false, refetchOnWindowFocus: false } },
+});
+
+ReactDOM.createRoot(document.getElementById("root")!).render(
+  <React.StrictMode>
+    <ThemeProvider>
+      <I18nProvider>
+        <QueryClientProvider client={queryClient}>
+          <BrowserRouter>
+            <App />
+          </BrowserRouter>
+        </QueryClientProvider>
+      </I18nProvider>
+    </ThemeProvider>
+  </React.StrictMode>,
+);