Stabilize browser WebSocket heartbeat

Refresh browser-side WebSocket activity tracking on valid inbound messages so normal server heartbeat traffic prevents unnecessary reconnect churn.

Cover the quiet-period timeout path with a regression test.

docs/release-control/v6/internal/subsystems/unified-resources.md

@@ -289,6 +289,10 @@ AI-only summary payloads, or page-local heuristics.
    from websocket `state.resources` instead of layering confirmatory
    route-local REST refetch loops over already-owned resource
    updates.
+   Browser WebSocket liveness tracking is part of that same store boundary:
+   valid inbound server messages, including heartbeat `ping`/`pong` traffic,
+   must refresh the browser-side activity timestamp so quiet periods between
+   resource snapshots do not cause avoidable reconnect churn.
    That shared store/adapter/hook path must also preserve canonical row shape
    across transport boundaries: thinner realtime `state.resources` payloads
    must merge into the existing canonical resource snapshot instead of

frontend-modern/src/stores/__tests__/websocket-resilience.test.ts

@@ -143,6 +143,25 @@ describe('websocket store resilience', () => {
     }
   });
 
+  it('keeps the websocket open when server activity arrives before heartbeat timeout', async () => {
+    const { dispose } = await createStoreHarness();
+    try {
+      vi.advanceTimersByTime(1); // run onopen tick
+      expect(currentInstance).not.toBeNull();
+
+      vi.advanceTimersByTime(89_000);
+      currentInstance!.onmessage?.({
+        data: JSON.stringify({ type: 'pong', data: { timestamp: Date.now() } }),
+      } as MessageEvent);
+      vi.advanceTimersByTime(2_000);
+
+      expect(currentInstance!.close).not.toHaveBeenCalledWith(4000, 'Heartbeat timeout');
+      expect(currentInstance!.close).not.toHaveBeenCalled();
+    } finally {
+      dispose();
+    }
+  });
+
   it('manual reconnect avoids duplicate reconnect scheduling', async () => {
     const { store, dispose } = await createStoreHarness();
     try {

frontend-modern/src/stores/websocket.ts

@@ -411,6 +411,7 @@ export function createWebSocketStore(url: string) {
         logger.error('Failed to parse WebSocket message', parseError);
         return;
       }
+      lastServerActivityAt = Date.now();
 
       try {
         const message = data as TimestampedWSMessage;