fix: resolve WebSocket connection issues and API errors

- Fix React StrictMode double-mount causing WebSocket connection loops
  - Add isMountedRef to Tasks.tsx and NotificationContext.tsx
  - Delay WebSocket connection by 100ms to avoid race conditions
  - Check mounted state before reconnection attempts

- Fix React Router v7 deprecation warnings
  - Add future flags: v7_startTransition, v7_relativeSplatPath

- Fix CalendarView 422 API error
  - Send full ISO 8601 datetime format instead of date-only
  - Add URL encoding for query parameters

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/Tasks.tsx

@@ -122,14 +122,28 @@ export default function Tasks() {
   }
 
   // Subscribe to project WebSocket when project changes
+  // Use isMounted ref to handle React StrictMode's double-mount behavior
+  const isMountedRef = useRef(true)
   useEffect(() => {
+    isMountedRef.current = true
     if (projectId) {
-      subscribeToProject(projectId)
+      // Small delay to avoid race conditions with StrictMode's rapid mount/unmount
+      const timeoutId = setTimeout(() => {
+        if (isMountedRef.current) {
+          subscribeToProject(projectId)
+        }
+      }, 100)
+      return () => {
+        clearTimeout(timeoutId)
+        isMountedRef.current = false
+        unsubscribeFromProject()
+      }
     }
     return () => {
-      unsubscribeFromProject()
+      isMountedRef.current = false
     }
-  }, [projectId, subscribeToProject, unsubscribeFromProject])
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [projectId])
 
   // Handle real-time task events from WebSocket
   const handleTaskEvent = useCallback((event: TaskEvent) => {