fix(web): prevent SSE effect from re-running on every mux snapshot

The SSE effect had muxSessions (array reference) in its deps, causing
it to re-run on every snapshot and fire cleanup that cleared the
debounce timer the mux effect intentionally preserved.

- Derive muxActive = muxSessions !== undefined and use that in SSE
  effect deps — the effect now only re-runs when mux transitions
  between present/absent, not on every new array reference
- Add reschedule in scheduleRefresh .finally() abort path: when a
  fetch is aborted mid-flight (by a new snapshot) and there is still
  a pending membership key, reschedule so the refresh isn't silently
  dropped

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

packages/web/src/hooks/useSessionEvents.ts

@@ -136,6 +136,11 @@ export function useSessionEvents(
     });
   }, [initialSessions, initialGlobalPause]);
 
+  // Stable boolean — only changes when mux transitions between present/absent,
+  // not on every new snapshot array reference. Used in the SSE effect deps so
+  // SSE setup/teardown runs only on that transition, not every mux update.
+  const muxActive = muxSessions !== undefined;
+
   // Define scheduleRefresh with useCallback so both effects can use it
   const scheduleRefresh = useCallback(() => {
     if (refreshingRef.current || refreshTimerRef.current) return;
@@ -174,6 +179,10 @@ export function useSessionEvents(
           }
           if (refreshController.signal.aborted) {
             refreshingRef.current = false;
+            // If there's still a pending membership change, reschedule so it isn't lost
+            if (pendingMembershipKeyRef.current !== null) {
+              scheduleRefresh();
+            }
             return;
           }
 
@@ -221,7 +230,7 @@ export function useSessionEvents(
 
   useEffect(() => {
     // Skip SSE if mux sessions are available
-    if (muxSessions) {
+    if (muxActive) {
       dispatch({ type: "setConnection", status: "connected" });
       return () => {
         // Clear any pending refresh timer so it doesn't fire after unmount
@@ -313,7 +322,7 @@ export function useSessionEvents(
       clearDisconnectedTimer();
       es.close();
     };
-  }, [project, muxSessions, scheduleRefresh]);
+  }, [project, muxActive, scheduleRefresh]);
 
   return state;
 }