feat: add workspace import UI flow

[neversettle17-101]

Summary

• adds the workspace/project choice flow from the sidebar add button
• lets users pick a folder, worker agent, and orchestrator agent before importing
• surfaces folder scan/validation details so workspace import failures can show detected repos and guidance

Stack

This PR is stacked on #2328 and targets ao/agent-orchestrator-19/workspace-backend-scm-observer because it depends on the workspace backend follow-up APIs/behavior.

Validation

• npm run frontend:typecheck
• npm --prefix frontend run test -- Sidebar.test.tsx

[neversettle17-101]

Closing duplicate. The UI changes have been moved to the existing UI PR #2315 and retargeted to the current backend stack.