Project import UI

## Scope

- New component: `ProjectImport.tsx` in `web/components/import-export/`.
- New RTK Query service: `useProjectImport.ts` in `common/services/`.
- `createProjectImport` mutation.
- `getProjectImports` query.
- New request/response types.
- Integrate into the existing Import tab on project settings page.
- Add a toggle or sub-tab: "Environment Import" (existing) / "Project Import" (new).
- UI elements:
- JSON file upload (reuse `JSONUpload` component).
- Merge strategy radio buttons (Skip / Overwrite Destructive).
- Warning message for Overwrite Destructive (reuse pattern from `FeatureImport.tsx`).
- "Import Project" button.
- Import status list with polling.

## Acceptance criteria

- User can upload a project export file and select a merge strategy.
- Warning shown for Overwrite Destructive.
- Import list shows status with polling.