Per-key icon overlay on assigned keys

Small icon displayed on assigned keys (key letter repositioned to top-left corner). Built-in SVG library (~50-80 icons covering CAD commands, productivity, arrows) from permissive-licence sets (Lucide, Heroicons, Phosphor). Icon ID stored per assignment. Right-click key, set icon, searchable picker. Purely cosmetic. Validate need from Alpha feedback before building.