feat(KB-662): add visibility change listener with debouncing to task refresh

- Add visibilitychange listener in useTasks hook to refresh tasks when tab becomes visible
- Implement 1-second debounce to prevent rapid refetching on visibility changes
- Add comprehensive tests for visibility change behavior including debouncing
- Clean up unused variable in SettingsModal from merge conflict resolution

packages/dashboard/app/components/SettingsModal.tsx

@@ -167,9 +167,6 @@ export function SettingsModal({
     };
   }, [activeSection, loadAuthStatus]);
 
-  /** Get the scope of the currently active section */
-  const activeSectionScope = SETTINGS_SECTIONS.find((s) => s.id === activeSection)?.scope;
-
   const handleLogin = useCallback(async (providerId: string) => {
     setAuthActionInProgress(providerId);
     try {