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>

docs/scratchpads/468-kanban-page.md

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+# PG-PAGE-003: Kanban Board Page
+
+## Task
+
+Build Kanban board page with drag-and-drop columns mapped to TaskStatus.
+
+## Issue
+
+Refs #468
+
+## Files Changed
+
+- `apps/web/src/app/(authenticated)/kanban/page.tsx` (new)
+- `apps/web/src/lib/api/tasks.ts` (added `updateTask`)
+- `package.json` / `pnpm-lock.yaml` (added `@hello-pangea/dnd`)
+
+## Design Decisions
+
+- Used `@hello-pangea/dnd` (maintained fork of react-beautiful-dnd) for drag-and-drop
+- 5 columns mapped to TaskStatus enum: NOT_STARTED, IN_PROGRESS, PAUSED, COMPLETED, ARCHIVED
+- Optimistic update pattern: move card immediately, revert by re-fetching on API failure
+- Priority badge always shown (field is non-optional in Task type)
+- Column count badge uses `color-mix()` for transparent accent backgrounds
+- Horizontal scroll on mobile with `overflow-x: auto` and `min-width: 280px` per column
+
+## Verification
+
+- Lint: clean (my files pass, pre-existing errors in runner-jobs.ts not my scope)
+- Build: `next build` succeeds, `/kanban` route present in output
+- TypeScript: no type errors
+- Design tokens: all colors from CSS custom properties, no hardcoded colors
+
+## Status
+
+Complete