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/KanbanColumn.tsx

@@ -1,5 +1,6 @@
 'use client';
 
+import React from 'react';
 import { useDroppable } from '@dnd-kit/core';
 import { SortableContext, verticalListSortingStrategy } from '@dnd-kit/sortable';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
@@ -14,7 +15,7 @@ interface KanbanColumnProps {
   taskCounts?: Record<string, number>; // Map of storyId -> taskCount
 }
 
-export function KanbanColumn({ id, title, stories, epicNames = {}, taskCounts = {} }: KanbanColumnProps) {
+export const KanbanColumn = React.memo(function KanbanColumn({ id, title, stories, epicNames = {}, taskCounts = {} }: KanbanColumnProps) {
   const { setNodeRef } = useDroppable({ id });
 
   return (
@@ -47,4 +48,4 @@ export function KanbanColumn({ id, title, stories, epicNames = {}, taskCounts =
       </CardContent>
     </Card>
   );
-}
+});