Fix: Check compute connection status before project deletion

This commit addresses issue #2703 where deleting a project with nodes
on remote compute nodes would result in long waits with no feedback
if those computes were unreachable.

Changes:
1. Compute connection status updates on connection failure
   - When a compute fails to connect, update connected=False and last_error
   - Send compute.updated notification to UI so users can see status
   - This allows Web UI to display real-time connection status

2. Project deletion checks compute status before attempting deletion
   - Check all computes used by the project are connected
   - If any compute is disconnected, immediately reject deletion
   - Provide clear error message indicating which computes are offline
   - This prevents long timeouts and gives users immediate feedback

Benefits:
- Immediate feedback instead of 120-second timeouts
- Clear error messages about which computes are disconnected
- Prevents orphaned resources on offline computes
- Improves user experience by avoiding silent waits

Related: #2703

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

gns3server/controller/compute.py

@@ -375,6 +375,11 @@ class Compute:
                 log.info(f"Connecting to compute '{self._id}'")
                 response = await self._run_http_query("GET", "/capabilities")
             except ComputeError as e:
+                # Update connection status and notify UI
+                self._connected = False
+                self._last_error = str(e)
+                self._controller.notification.controller_emit("compute.updated", self.asdict())
+
                 if report_failed_connection:
                     raise
                 log.warning(f"Cannot connect to compute '{self._id}': {e}")