Fix "quick start" file page overflow when file tree is longer than height

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/33273

Description

This PR:

• Fix quick start tour causing overflow on the file page

Test plan

• Run sourcegraph web locally: SOURCEGRAPH_API_URL=https://sourcegraph.com SOURCEGRAPHDOTCOM_MODE=true sg -skip-auto-update=true start web-standalone
• Open https://sourcegraph.test:3443/github.com/sourcegraph/sourcegraph/-/blob/enterprise/cmd/frontend/internal/batches/resolvers/urls.go?feature-flag-key=quick-start-tour-for-authenticated-users&feature-flag-value=true in a regular small desktop screen
• Check that page is rendered correctly, tour is shown and File tree section auto-scrolls to the selected file
• Close the tour and refresh. Check that page is rendered correctly, file tree section auto-scrolls to the selected file