fix(shortcuts): restore cmd customization + degrade gracefully on Wayland
Cmd shortcut customization went missing when cmd moved into core. This
restores it via a control in the core Keyboard Shortcuts group. Also:
local-fallback shortcuts for peeks/slides, quieted Wayland warnings,
api.platform exposed to renderers, and Linux default switched to
CommandOrControl+Space (Alt+Space conflicts with GNOME).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>