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/components/CalendarView.tsx

@@ -119,12 +119,12 @@ export function CalendarView({
   const loadTasks = async (start: Date, end: Date) => {
     setLoading(true)
     try {
-      // Format dates for API
-      const dueAfter = start.toISOString().split('T')[0]
-      const dueBefore = end.toISOString().split('T')[0]
+      // Format dates for API (backend expects ISO 8601 datetime)
+      const dueAfter = start.toISOString()
+      const dueBefore = end.toISOString()
 
       const response = await api.get(
-        `/projects/${projectId}/tasks?due_after=${dueAfter}&due_before=${dueBefore}`
+        `/projects/${projectId}/tasks?due_after=${encodeURIComponent(dueAfter)}&due_before=${encodeURIComponent(dueBefore)}`
       )
       const tasks: Task[] = response.data.tasks