feat(web): add kanban board page with drag-and-drop (#478)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.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