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>

status/css/dashboard.css

@@ -1964,6 +1964,22 @@ button:focus-visible {
   cursor: wait;
 }
 
+/* Exec/terminal button styling — subtle, hover-only */
+.exec-btn {
+  margin-left: 4px !important;
+  font-size: .7rem !important;
+  font-family: monospace !important;
+  padding: .2rem .45rem !important;
+  opacity: 0;
+  transition: opacity 0.2s ease;
+}
+.card:hover .exec-btn {
+  opacity: 0.6;
+}
+.exec-btn:hover {
+  opacity: 1 !important;
+}
+
 /* Credentials (key) button styling */
 .creds-btn {
   margin-right: 8px !important;