feat(auth): redesign Login and Setup pages with split-panel branding layout (#153)

Replace plain Card-based Login and Setup forms with a professional
split-panel layout: always-dark branding panel (dot grid texture, logo,
tagline, cyan accent line) on the left, theme-aware form on the right.
Mobile collapses to single column with compact logo header.

- Add optional admin email field on Setup for license recovery
- Backend accepts and stores admin_email in setup endpoint
- Fix data-stacks-loaded attribute forwarding (wrap in div, use string values)

backend/src/index.ts

@@ -277,7 +277,7 @@ app.post('/api/auth/setup', authRateLimiter, async (req: Request, res: Response)
       return;
     }
 
-    const { username, password, confirmPassword } = req.body;
+    const { username, password, confirmPassword, admin_email } = req.body;
 
     // Validation
     if (!username || !password || !confirmPassword) {
@@ -307,6 +307,10 @@ app.post('/api/auth/setup', authRateLimiter, async (req: Request, res: Response)
     dbSvc.updateGlobalSetting('auth_password_hash', passwordHash);
     dbSvc.updateGlobalSetting('auth_jwt_secret', jwtSecret);
 
+    if (admin_email && typeof admin_email === 'string') {
+      dbSvc.updateGlobalSetting('admin_email', admin_email.trim());
+    }
+
     // Issue JWT and log user in
     const token = jwt.sign({ username }, jwtSecret, { expiresIn: '24h' });
     res.cookie(COOKIE_NAME, token, getCookieOptions(req));