Relocate frontend source code from `next-app/` to `frontend/` to align with the new project structure. This includes removing the old Next.js boilerplate files and establishing a cleaner workspace.
Additionally, updates the OpenAPI specification to include support for the `immich` widget type and its corresponding configuration schema.
- Move frontend files to `frontend/`
- Delete obsolete `next-app/` directory and its configuration
- Add `immich` widget type to `openapi.yaml`
- Update `FrontendPlan.md` with dashboard refactor and UX direction
Tomas Dvorak