IDE and host settings
Added a popup settings page for enabling/disabling IDEs and GitHub hosts.

Custom IDE support
Users can add custom IDE launchers with a display name, URI template, and optional SVG icon.

Custom GitHub hosts support
Users can add custom GitHub hosts (*.ghe.com).

Improved 'Open With' dialog
The IDE picker now uses a selection-first dialog with 'Open once' and 'Open always' actions.

Improved keyboard navigation
The 'Open With' dialog now supports Arrow Up/Down navigation and Enter to open.

SSH clone URL support
Buttons now work when the GitHub clone picker is set to SSH (previously only HTTPS was recognised).

Dynamic URL resolution
Repo URL is resolved at click time, so switching between HTTPS/SSH after buttons appear is handled correctly.

IDE validation
Stored IDE ID is now validated against the supported list; invalid values fall back to idea.

Async/await refactor
getDefaultIDE rewritten for cleaner error handling and a more reliable localStorage fallback.