Fix event lock system: banner persistence, stale isEditing guard, and Edit button gating

- Remove isEditing guard from viewLockQuery effect so lock banner shows for user B
  even after user A transitions into edit mode (fixes banner disappearing)
- Disable Edit button proactively when lockInfo.locked is already known from polling,
  preventing the user from even attempting acquireLock when a lock is active
- Fix acquire callback dep array in useEventLock (missing acquireMutation)

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

frontend/src/components/calendar/EventDetailPanel.tsx

@@ -276,16 +276,15 @@ export default function EventDetailPanel({
     refetchInterval: 5_000,
   });
 
-  // Show/hide lock banner proactively in view mode
+  // Show/hide lock banner proactively in view mode (poll data always authoritative)
   useEffect(() => {
-    if (viewLockQuery.data && !isEditing && !isCreating) {
+    if (!viewLockQuery.data) return;
     if (viewLockQuery.data.locked) {
       setLockInfo(viewLockQuery.data);
     } else {
       setLockInfo(null);
     }
-    }
-  }, [viewLockQuery.data, isEditing, isCreating]);
+  }, [viewLockQuery.data]);
 
   const isRecurring = !!(event?.is_recurring || event?.parent_event_id);
 
@@ -581,8 +580,8 @@ export default function EventDetailPanel({
                         size="icon"
                         className="h-7 w-7"
                         onClick={handleEditStart}
-                        disabled={isAcquiringLock}
-                        title="Edit event"
+                        disabled={isAcquiringLock || !!(lockInfo && lockInfo.locked)}
+                        title={lockInfo && lockInfo.locked ? `Locked by ${lockInfo.locked_by_name || 'another user'}` : 'Edit event'}
                       >
                         {isAcquiringLock ? <Loader2 className="h-3.5 w-3.5 animate-spin" /> : <Pencil className="h-3.5 w-3.5" />}
                       </Button>

frontend/src/hooks/useEventLock.ts

@@ -34,7 +34,7 @@ export function useEventLock(eventId: number | null) {
     if (!eventId) return null;
     const data = await acquireMutation.mutateAsync(eventId);
     return data;
-  }, [eventId]);
+  }, [eventId, acquireMutation]);
 
   const release = useCallback(async () => {
     const id = activeEventIdRef.current;