v2.3 - Mobile Task Cards

What changed

Frontend

• Task list now shows as compact cards on mobile (below 768px) instead of a horizontal-scrolling table
• Each card shows a circle checkbox, task title (links to detail page), project name, deadline with proximity colour, and a three-dot menu with a Delete action
• Desktop table layout is unchanged
• Shared date formatting utilities extracted to src/lib/format.ts (closes backlog item #20)