Summary

• Adds a 🖨️ Print button in the dashboard navbar that opens the browser print dialog via window.print()
• Adds print-only CSS in index.css to hide chrome UI (.no-print) and keep task cards from splitting across pages
• Marks the kanban container as .board and task cards as .card so print styles apply

Closes #388

Test plan

• Open the dashboard with tasks in multiple columns
• Click 🖨️ Print — browser print preview opens
• Confirm search, heatmap, pomodoro, and action buttons are hidden in print preview
• Confirm board columns/tasks are visible and cards are not awkwardly split across pages
• Cancel print — app UI remains unchanged