fix: preserve physical disk data for temporarily offline nodes (addresses #429)

When a node is temporarily offline or has connectivity issues, preserve its last known physical disk data instead of removing it from the table. This prevents nodes from disappearing from the Physical Disks table when they have transient connection problems.

internal/monitoring/monitor.go

@@ -943,11 +943,23 @@ func (m *Monitor) pollPVEInstance(ctx context.Context, instanceName string, clie
 
 	// Poll physical disks for health monitoring
 	log.Debug().Int("nodeCount", len(nodes)).Msg("Starting disk health polling")
+	
+	// Get existing disks from state to preserve data for offline nodes
+	currentState := m.state.GetSnapshot()
+	existingDisksMap := make(map[string]models.PhysicalDisk)
+	for _, disk := range currentState.PhysicalDisks {
+		if disk.Instance == instanceName {
+			existingDisksMap[disk.ID] = disk
+		}
+	}
+	
 	var allDisks []models.PhysicalDisk
+	polledNodes := make(map[string]bool) // Track which nodes we successfully polled
+	
 	for _, node := range nodes {
-		// Skip offline nodes
+		// Skip offline nodes but preserve their existing disk data
 		if node.Status != "online" {
-			log.Debug().Str("node", node.Node).Msg("Skipping disk poll for offline node")
+			log.Debug().Str("node", node.Node).Msg("Skipping disk poll for offline node - preserving existing data")
 			continue
 		}
 		
@@ -968,6 +980,9 @@ func (m *Monitor) pollPVEInstance(ctx context.Context, instanceName string, clie
 			Int("diskCount", len(disks)).
 			Msg("Got disk list for node")
 		
+		// Mark this node as successfully polled
+		polledNodes[node.Node] = true
+		
 		// Check each disk for health issues and add to state
 		for _, disk := range disks {
 			// Create PhysicalDisk model
@@ -1025,10 +1040,24 @@ func (m *Monitor) pollPVEInstance(ctx context.Context, instanceName string, clie
 		}
 	}
 	
+	// Preserve existing disk data for nodes that weren't polled (offline or error)
+	for diskID, existingDisk := range existingDisksMap {
+		// Only preserve if we didn't poll this node
+		if !polledNodes[existingDisk.Node] {
+			// Keep the existing disk data but update the LastChecked to indicate it's stale
+			allDisks = append(allDisks, existingDisk)
+			log.Debug().
+				Str("node", existingDisk.Node).
+				Str("disk", existingDisk.DevPath).
+				Msg("Preserving existing disk data for unpolled node")
+		}
+	}
+	
 	// Update physical disks in state
 	log.Debug().
 		Str("instance", instanceName).
 		Int("diskCount", len(allDisks)).
+		Int("preservedCount", len(existingDisksMap)-len(polledNodes)).
 		Msg("Updating physical disks in state")
 	m.state.UpdatePhysicalDisks(instanceName, allDisks)