Keyboard shortcuts for common actions

[qapitall]

What

A few global keyboard shortcuts:

• n — open the New card modal (To Do)
• / — focus the plan input
• Esc — close whichever modal is open

Why

The app is fast with a mouse, but daily-driver tools live on the keyboard. Right now even closing a modal requires clicking Cancel.

Suggested approach

• public/app.js — one keydown listener on document. Ignore keypresses when the target is an input, textarea or select (except Esc, which should always work). n opens #card-modal the same way the column "+" buttons do; Esc hides #card-modal / #preview-modal if visible.
• Optionally list the shortcuts in the README.

Acceptance criteria

• n opens the new-card modal, / focuses the plan box, Esc closes open modals.
• Shortcuts don't fire while typing in a field.
• Esc works even while a field inside a modal has focus.

Good first issue — frontend only, one event listener.