[MekDrop]

Summary

• Replaces the stub ExtractPage.vue with a full implementation
• Displays the selected source repo with a branch selector; loads the file tree via GitHub/GitLab APIs and renders it through FolderTree
• Provides target repo selection (with live filter), target branch selection, and target path input
• On submit, POSTs an extract job to WORKER_URL/extract and shows a success banner
• Supports both GitHub and GitLab providers via the auth store

Test plan

• Select a source repo on the Repos page and navigate to Extract
• Verify branch selector populates and switching branches reloads the tree
• Verify folder tree renders and selecting a path updates store.sourcePath
• Select a target repo and verify branch selector populates with the repo's branches
• Verify "Extract → Push" button is disabled until source path, target repo, and target branch are all set
• Submit and verify the success banner appears

Closes #12