feat(frontend): workspace/project import UI flow

[neversettle17-101]

Summary

• add the import modal type picker for Workspace vs Project from the existing sidebar add-project entry point
• add Electron IPC folder scanning for project/workspace repo previews, including branch and origin remote validation
• submit workspace imports through POST /api/v1/projects with asWorkspace:true and project imports with asWorkspace:false while preserving worker/orchestrator agent selection

Testing

• cd frontend && npm run typecheck
• cd frontend && npm test -- Sidebar.test.tsx
• cd frontend && npm test

References #2222.

Stacked on #2243 so this branch includes the backend workspace project and lifecycle changes for end-to-end testing.