Move GitHub connection and issue-linking flows into modals

Summary

Replace the inline GitHub connection controls with modal-based flows in both places where they appear.

Requirements

• On the project About page, adding a GitHub connection should open in a modal instead of rendering an inline form.
• On an issue, linking a repository and branch should open in a modal instead of rendering inline controls.
• Reuse the existing shared modal and form patterns so both flows feel consistent.
• Preserve current validation, loading, success, and error behavior.
• After a successful save, close the modal and refresh the relevant connection/link state.
• Support normal modal accessibility behavior, including initial focus, keyboard navigation, Escape to close, and restoring focus to the trigger.

Acceptance criteria

• Neither surface expands an inline GitHub form.
• Each trigger opens the appropriate modal and can be cancelled without changing data.
• Successful submissions update the page state and close the modal.
• Validation and server errors remain visible within the modal.