feat: add 7 new features — exec shell, SSE events, compose import, docker resources, resource limits, email notifications, auto-updates

- Container exec/shell via WebSocket + xterm.js (subtle >_ button on cards)
- Live dashboard updates via SSE (resource alerts, health changes, update notices)
- Docker Compose import with YAML parsing, preview, and dependency-ordered deploy
- Volume & network management modal with disk usage overview
- CPU/memory resource limits on deploy and live update
- Email SMTP notifications (nodemailer) alongside Discord/Telegram/ntfy
- Scheduled auto-update scheduler with maintenance windows (daily/weekly/monthly)

New deps: ws, js-yaml, nodemailer

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-api/routes/updates.js

@@ -59,6 +59,12 @@ module.exports = function({ updateManager, selfUpdater, asyncHandler, logError }
     res.json({ success: true, message: 'Auto-update configured' });
   }, 'updates-auto-update'));
 
+  // Get auto-update configuration
+  router.get('/updates/auto-update', asyncHandler(async (req, res) => {
+    const config = updateManager.getAutoUpdateConfig();
+    res.json({ success: true, config });
+  }, 'updates-auto-update-config'));
+
   // Schedule update
   router.post('/updates/schedule/:containerId', asyncHandler(async (req, res) => {
     const { scheduledTime } = req.body;