Customizable keyboard shortcuts

[Oaklight]

Problem

Shortcut bindings are hardcoded. Users with conflicting OS shortcuts or different muscle memory can't adjust.

Proposed

• Settings panel: list of all actions with current binding, click to rebind
• Persist to user config (~/.config/tinyleaf/keymap.json)
• Reset-to-default button

Notes

Frontend keymap layer needs to be data-driven first. Modest refactor before this is feasible.

Priority: P3