Multi-repo support — pick destination repo when saving

Right now Magpie commits to a single configured repo. Many users want: tech articles → repo A, life notes → repo B. Pick at save time via dropdown.

Implementation sketch:

• Settings becomes Settings[] + defaultRepoIndex
• Options page gains a repo list manager (add/edit/delete)
• Popup gains a repo dropdown next to Save button
• Optional: auto-route by URL hostname