fix: apply empty agent updates immediately when existing agents are stale

The anti-flapping logic previously required 3 consecutive empty updates
before clearing stale agents from the UI. Now if all existing hosts/dockerHosts
have lastSeen > 60 seconds ago, empty updates are applied immediately.

This fixes the issue where removed agents stayed visible for too long.

frontend-modern/src/stores/websocket.ts

@@ -443,14 +443,25 @@ export function createWebSocketStore(url: string) {
                 if (incomingHosts.length === 0) {
                   consecutiveEmptyDockerUpdates += 1;
 
+                  // Check if all existing docker hosts are stale (>60s since lastSeen)
+                  // If so, they're probably really gone - apply the empty update immediately
+                  const now = Date.now();
+                  const staleThresholdMs = 60_000; // 60 seconds
+                  const existingDockerHosts = state.dockerHosts || [];
+                  const allStale = existingDockerHosts.length === 0 || existingDockerHosts.every(
+                    (h) => !h.lastSeen || (now - h.lastSeen) > staleThresholdMs
+                  );
+
                   shouldApplyDockerHosts =
                     !hasReceivedNonEmptyDockerHosts ||
+                    allStale ||
                     consecutiveEmptyDockerUpdates >= 3 ||
                     message.type === WEBSOCKET.MESSAGE_TYPES.INITIAL_STATE;
 
                   if (shouldApplyDockerHosts) {
                     logger.debug('[WebSocket] Updating dockerHosts', {
                       count: incomingHosts.length,
+                      reason: allStale ? 'allStale' : 'threshold',
                     });
                     processedDockerHosts = mergeDockerHostRevocations(incomingHosts);
                   } else {
@@ -486,14 +497,25 @@ export function createWebSocketStore(url: string) {
                 if (incomingHosts.length === 0) {
                   consecutiveEmptyHostUpdates += 1;
 
+                  // Check if all existing hosts are stale (>60s since lastSeen)
+                  // If so, they're probably really gone - apply the empty update immediately
+                  const now = Date.now();
+                  const staleThresholdMs = 60_000; // 60 seconds
+                  const existingHosts = state.hosts || [];
+                  const allHostsStale = existingHosts.length === 0 || existingHosts.every(
+                    (h) => !h.lastSeen || (now - h.lastSeen) > staleThresholdMs
+                  );
+
                   shouldApplyHosts =
                     !hasReceivedNonEmptyHosts ||
+                    allHostsStale ||
                     consecutiveEmptyHostUpdates >= 3 ||
                     message.type === WEBSOCKET.MESSAGE_TYPES.INITIAL_STATE;
 
                   if (shouldApplyHosts) {
                     logger.debug('[WebSocket] Updating hosts', {
                       count: incomingHosts.length,
+                      reason: allHostsStale ? 'allStale' : 'threshold',
                     });
                     processedHosts = mergeHostRevocations(incomingHosts);
                   } else {