Fix: clamp git actions menu to viewport

[Finesssee]

Fixes #553

Summary

• anchor the Source Control actions menu to the trigger button instead of raw click coordinates
• clamp the menu inside the viewport and open upward when there is not enough room below
• add focused position helper tests for edge and corner placements

Testing

• bun fix
• bun typecheck
• bun test

[mehmetozguldev]

Reviewed locally. Conflict resolved by preserving the viewport-clamping anchor logic while keeping the shared ContextMenu integration from master. Targeted tests and typecheck passed.