fix: xterm.js CDN → jsdelivr, port 8000 → 7272, custom port in installer

- Switch xterm.js from cdnjs (404) to jsdelivr (working)
- Add xterm-addon-fit for auto-resize terminal
- Change default port from 8000 to 7272 everywhere
- Install script: --port flag for custom port selection
- sed-based port injection into systemd service at install time

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

server/templates/dashboard.html

@@ -5,7 +5,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>VPS Monitoring</title>
     <link rel="stylesheet" href="/static/css/style.css">
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/xterm/5.3.0/xterm.min.css">
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/xterm@5.3.0/css/xterm.min.css">
     <script src="/static/js/theme.js"></script>
     <script src="/static/js/i18n.js"></script>
 </head>
@@ -165,7 +165,8 @@
         </div>
     </div>
 
-    <script src="https://cdnjs.cloudflare.com/ajax/libs/xterm/5.3.0/xterm.min.js"></script>
+    <script src="https://cdn.jsdelivr.net/npm/xterm@5.3.0/lib/xterm.min.js"></script>
+    <script src="https://cdn.jsdelivr.net/npm/xterm-addon-fit@0.8.0/lib/xterm-addon-fit.min.js"></script>
     <script src="/static/js/app.js"></script>
 </body>
 </html>