Ability to view all keyboard shortcuts

It would be nice to have the ability where users can press ? (or another key) anytime to see all available shortcuts organized by category, making it:

- Easy to learn shortcuts systematically
- Quick to reference when needed
- Self-documenting (no external docs needed)

Start by locating the existing keyboard-shortcut handling and the UI entry point for displaying overlays or help. Confirm how shortcuts are currently defined, then implement a categorized view opened by ? or another key. Done means users can open it anytime, see all available shortcuts grouped by category, and dismiss it without disrupting their work.

Written by the indexing model from the issue text.