feat(web): add kanban board page with drag-and-drop

- Add kanban board page at /kanban with 5 status columns
  (To Do, In Progress, Paused, Done, Archived)
- Use @hello-pangea/dnd for drag-and-drop between columns
- Optimistic status update on drop with API revert on failure
- Add updateTask() to tasks API client
- Task cards show title, priority badge, and truncated description
- Column headers with colored accent borders and task count badges
- Responsive: horizontal scroll with min-width 280px per column
- Loading (MosaicSpinner), error, and empty states
- All design tokens from CSS custom properties

Refs #468

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/package.json

@@ -18,6 +18,7 @@
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/sortable": "^9.0.0",
     "@dnd-kit/utilities": "^3.2.2",
+    "@hello-pangea/dnd": "^18.0.1",
     "@mosaic/shared": "workspace:*",
     "@mosaic/ui": "workspace:*",
     "@tanstack/react-query": "^5.90.20",