perf(frontend): Optimize component rendering with React.memo and hooks - Sprint 3 Story 2

Add React.memo to display components and useCallback/useMemo for better performance.

Changes:
- Added React.memo to TaskCard component
- Added React.memo to StoryCard component
- Added React.memo to KanbanBoard component
- Added React.memo to KanbanColumn component
- Added useCallback to kanban page drag handlers (handleDragStart, handleDragEnd)
- Added useCallback to epics page handlers (handleDelete, getStatusColor, getPriorityColor)
- Added useMemo for expensive computations in dashboard page (stats, recentProjects sorting)
- Added useMemo for total tasks calculation in KanbanBoard
- Removed unused isConnected variable from kanban page

Performance improvements:
- Reduced unnecessary re-renders in Card components
- Optimized list rendering performance with memoized callbacks
- Improved filtering and sorting performance with useMemo
- Better React DevTools Profiler metrics

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

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

components/features/kanban/TaskCard.tsx

@@ -1,5 +1,6 @@
 'use client';
 
+import React from 'react';
 import { Card, CardContent, CardHeader } from '@/components/ui/card';
 import { Clock, User } from 'lucide-react';
 import type { TaskCard as TaskCardType } from '@/types/kanban';
@@ -9,7 +10,7 @@ interface TaskCardProps {
   isDragging?: boolean;
 }
 
-export function TaskCard({ task, isDragging = false }: TaskCardProps) {
+export const TaskCard = React.memo(function TaskCard({ task, isDragging = false }: TaskCardProps) {
   const priorityColors = {
     Low: 'bg-blue-100 text-blue-700',
     Medium: 'bg-yellow-100 text-yellow-700',
@@ -59,4 +60,4 @@ export function TaskCard({ task, isDragging = false }: TaskCardProps) {
       </CardContent>
     </Card>
   );
-}
+});