feat(frontend): Implement Issue management and Kanban board

Add comprehensive Issue management functionality with drag-and-drop Kanban board.

Changes:
- Created Issue API client (issues.ts) with CRUD operations
- Implemented React Query hooks for Issue data management
- Added IssueCard component with drag-and-drop support using @dnd-kit
- Created KanbanColumn component with droppable zones
- Built CreateIssueDialog with form validation using zod
- Implemented Kanban page at /projects/[id]/kanban with DnD status changes
- Added missing UI components (textarea, select, skeleton)
- Enhanced API client with helper methods (get, post, put, patch, delete)
- Installed dependencies: @dnd-kit/core, @dnd-kit/sortable, @dnd-kit/utilities, @radix-ui/react-select, sonner
- Fixed SignalR ConnectionManager TypeScript error
- Preserved legacy KanbanBoard component for backward compatibility

Features:
- Drag and drop issues between Backlog, Todo, InProgress, and Done columns
- Real-time status updates via API
- Issue creation with type (Story, Task, Bug, Epic) and priority
- Visual feedback with priority colors and type icons
- Toast notifications for user actions

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

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

components/ui/skeleton.tsx

@@ -0,0 +1,15 @@
+import { cn } from '@/lib/utils';
+
+function Skeleton({
+  className,
+  ...props
+}: React.HTMLAttributes<HTMLDivElement>) {
+  return (
+    <div
+      className={cn('animate-pulse rounded-md bg-muted', className)}
+      {...props}
+    />
+  );
+}
+
+export { Skeleton };