fix: address code review feedback - add explicit TypeScript return types

- Add explicit JSX.Element return types to all Kanban components
- Add explicit void return type to handleDragStart
- Add explicit Promise<void> return type to handleDragEnd (async)
- Import React for JSX namespace access
- Complies with typescript.md: explicit return types REQUIRED

Components updated:
- KanbanBoard.tsx
- KanbanColumn.tsx
- TaskCard.tsx

Per code review checklist (code-review.md section 4a):
✓ NO any types
✓ Explicit return types on all exported functions
✓ Explicit parameter types
✓ Interfaces for props
✓ Proper event handler types

apps/web/src/components/kanban/TaskCard.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import React from "react";
 import type { Task } from "@mosaic/shared";
 import { TaskPriority } from "@mosaic/shared";
 import { useSortable } from "@dnd-kit/sortable";
@@ -47,7 +48,7 @@ function getInitials(name: string): string {
  * - Assignee avatar (if assigned)
  * - Due date (if set)
  */
-export function TaskCard({ task }: TaskCardProps) {
+export function TaskCard({ task }: TaskCardProps): JSX.Element {
   const {
     attributes,
     listeners,