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/contexts/NotificationContext.tsx

@@ -25,6 +25,7 @@ export function NotificationProvider({ children }: { children: ReactNode }) {
   const wsRef = useRef<WebSocket | null>(null)
   const pingIntervalRef = useRef<ReturnType<typeof setInterval> | null>(null)
   const reconnectTimeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null)
+  const isMountedRef = useRef(true)
 
   const refreshUnreadCount = useCallback(async () => {
     try {
@@ -168,10 +169,14 @@ export function NotificationProvider({ children }: { children: ReactNode }) {
           pingIntervalRef.current = null
         }
 
-        // Attempt to reconnect after delay
-        reconnectTimeoutRef.current = setTimeout(() => {
-          connectWebSocket()
-        }, WS_RECONNECT_DELAY)
+        // Only attempt to reconnect if component is still mounted
+        if (isMountedRef.current) {
+          reconnectTimeoutRef.current = setTimeout(() => {
+            if (isMountedRef.current) {
+              connectWebSocket()
+            }
+          }, WS_RECONNECT_DELAY)
+        }
       }
 
       ws.onerror = (err) => {
@@ -184,23 +189,35 @@ export function NotificationProvider({ children }: { children: ReactNode }) {
 
   // Initial fetch and WebSocket connection
   useEffect(() => {
+    isMountedRef.current = true
     const token = localStorage.getItem('token')
     if (token) {
       refreshUnreadCount()
-      connectWebSocket()
+      // Delay WebSocket connection to avoid StrictMode race condition
+      const connectTimeout = setTimeout(() => {
+        if (isMountedRef.current) {
+          connectWebSocket()
+        }
+      }, 100)
+
+      return () => {
+        clearTimeout(connectTimeout)
+        isMountedRef.current = false
+        // Cleanup on unmount
+        if (wsRef.current) {
+          wsRef.current.close()
+        }
+        if (pingIntervalRef.current) {
+          clearInterval(pingIntervalRef.current)
+        }
+        if (reconnectTimeoutRef.current) {
+          clearTimeout(reconnectTimeoutRef.current)
+        }
+      }
     }
 
     return () => {
-      // Cleanup on unmount
-      if (wsRef.current) {
-        wsRef.current.close()
-      }
-      if (pingIntervalRef.current) {
-        clearInterval(pingIntervalRef.current)
-      }
-      if (reconnectTimeoutRef.current) {
-        clearTimeout(reconnectTimeoutRef.current)
-      }
+      isMountedRef.current = false
     }
   }, [refreshUnreadCount, connectWebSocket])