Kanata: Expose layer-toggle and layer-switch in UI

[malpern]

Problem

Kanata supports layer-toggle (sticky on/off) and layer-switch (persistent switching) but KeyPath doesn't expose these in UI.

Current State

• Only momentary layer activation supported
• No UI for toggle/switch modes

Impact

Users can't create sticky layers or persistent layer switching via GUI.

Fix

1. Add layer-toggle and layer-switch to action types
2. Add UI controls for these modes
3. Generate correct Kanata syntax

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 4