Add VPN detection: show blocker when not on home IP

Check public IP via api.ipify.org on mount and display
"Выключи VPN" fullscreen message if it doesn't match
the expected home IP. Gracefully falls through on fetch errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.css

@@ -159,3 +159,13 @@ h2 {
 .actions button.danger:hover {
   background: #e74c3c;
 }
+
+.vpn-block {
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  height: 100vh;
+  font-size: 2rem;
+  font-family: system-ui, -apple-system, sans-serif;
+  color: #333;
+}