Step 14 - polish pass

Do a polish pass over the whole kanban feature.

1. Empty states: boards list, empty column, no search/filter results, empty trash - each with one clear next action.
2. Accessibility: keyboard operability for the task panel (focus trap, Escape closes, focus returns to the card), aria labels on icon-only buttons, visible focus states, color contrast to WCAG AA in BOTH themes; drag-and-drop gets a keyboard alternative (a "Move to column" menu on each card).
3. Dark theme audit: hunt hard-coded hex colors and replace with the CSS custom properties.
4. i18n: extract every user-facing string into a translation layer keyed by namespace (kanban.*, kanbanForms.*), even if only one language ships now.
5. Resilience: after a deploy, a stale client can fail to lazy-load a chunk ("Failed to fetch dynamically imported module") - add a one-time auto-reload handler for that error.
6. Add loading skeletons for board and panel, and confirm the missing/deleted-document states render on every route.