Refactor layout and home page for improved localization and session handling

- Updated RootLayout to include locale support and integrated ThemeProvider and LocaleProvider for better internationalization.
- Replaced static home page content with a redirect based on user session status, enhancing user experience by directing to the appropriate dashboard or login page.
- Switched font from Geist to Inter for improved typography consistency.

apps/webui/src/app/api/auth/callback/route.ts

@@ -0,0 +1,60 @@
+import type { SessionUser } from '@nexumi/shared';
+import { type NextRequest, NextResponse } from 'next/server';
+import { exchangeCodeForToken, fetchDiscordUser } from '@/lib/discord-oauth';
+import { env } from '@/lib/env';
+import { redis } from '@/lib/redis';
+import { createSession } from '@/lib/session';
+
+function oauthStateKey(state: string): string {
+  return `webui:oauth:state:${state}`;
+}
+
+function loginRedirect(error: string): NextResponse {
+  const url = new URL('/login', env.WEBUI_URL);
+  url.searchParams.set('error', error);
+  return NextResponse.redirect(url);
+}
+
+export async function GET(request: NextRequest) {
+  const { searchParams } = new URL(request.url);
+  const code = searchParams.get('code');
+  const state = searchParams.get('state');
+  const oauthError = searchParams.get('error');
+
+  if (oauthError) {
+    return loginRedirect('access_denied');
+  }
+
+  if (!code || !state) {
+    return loginRedirect('invalid_request');
+  }
+
+  const stateKey = oauthStateKey(state);
+  const stateExists = await redis.get(stateKey);
+  if (!stateExists) {
+    return loginRedirect('invalid_state');
+  }
+  await redis.del(stateKey);
+
+  try {
+    const token = await exchangeCodeForToken(code);
+    const discordUser = await fetchDiscordUser(token.access_token);
+
+    const user: SessionUser = {
+      id: discordUser.id,
+      username: discordUser.username,
+      globalName: discordUser.global_name ?? null,
+      avatar: discordUser.avatar ?? null
+    };
+
+    await createSession({
+      user,
+      accessToken: token.access_token,
+      tokenExpiresAt: Date.now() + token.expires_in * 1000
+    });
+  } catch {
+    return loginRedirect('oauth_failed');
+  }
+
+  return NextResponse.redirect(new URL('/dashboard', env.WEBUI_URL));
+}