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/KanbanBoard.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useMemo } from "react";
+import React, { useState, useMemo } from "react";
 import type { Task } from "@mosaic/shared";
 import { TaskStatus } from "@mosaic/shared";
 import {
@@ -40,7 +40,7 @@ const columns = [
  * - Task cards with title, priority badge, assignee avatar
  * - PATCH /api/tasks/:id on status change
  */
-export function KanbanBoard({ tasks = [], onStatusChange }: KanbanBoardProps) {
+export function KanbanBoard({ tasks = [], onStatusChange }: KanbanBoardProps): JSX.Element {
   const [activeTaskId, setActiveTaskId] = useState<string | null>(null);
 
   const sensors = useSensors(
@@ -80,11 +80,11 @@ export function KanbanBoard({ tasks = [], onStatusChange }: KanbanBoardProps) {
     [tasks, activeTaskId]
   );
 
-  function handleDragStart(event: DragStartEvent) {
+  function handleDragStart(event: DragStartEvent): void {
     setActiveTaskId(event.active.id as string);
   }
 
-  async function handleDragEnd(event: DragEndEvent) {
+  async function handleDragEnd(event: DragEndEvent): Promise<void> {
     const { active, over } = event;
 
     if (!over) {