Serve static files at root and redirect /explorer to /

## Parent PRD

- Parent #1618

## What to build

Change the Express server to serve the React client static files at `/` instead of `/explorer`. Add redirect middleware so that requests to `/explorer` (and `/explorer/*`) redirect to `/` (and `/*`). Update `GRAPH_EXP_ENV_ROOT_FOLDER` default from `/explorer` to `/`. Update the Dockerfile and SageMaker variant accordingly.

**Type: HITL** — requires review before completion.

## Acceptance criteria

- [ ] Static files are served at `/` instead of `/explorer`
- [ ] Requests to `/explorer` redirect (301) to `/`
- [ ] Requests to `/explorer/some/path` redirect to `/some/path`
- [ ] `GRAPH_EXP_ENV_ROOT_FOLDER` defaults to `/` in the Dockerfile
- [ ] SageMaker variant uses `/proxy/9250/` instead of `/proxy/9250/explorer`
- [ ] Vite config `base` default changes from `/explorer` to `/`
- [ ] Tests verify the redirect behavior

## Blocked by

- Blocked by #1619

## User stories addressed

- User story 7
- User story 14