bext: fix after-install page

Created by: erzhtor

Description

Seems like, after https://github.com/sourcegraph/sourcegraph/pull/30534, the browser extension after-install page was broken due to the Link is not set for it.

As long terms plan https://github.com/sourcegraph/sourcegraph/issues/31046, we will be rewriting browser extension storybook stories to replicate production configuration because currently it is wrapped into BrandedStory and other components that set the environment.

How to test

• sg run bext
• Reinstall browser extension locally and make sure that after install page is rendered.

Before merging

• Test on different code hosts (if applicable)

  • GitHub
  • Gitlab
  • GitHub Enterprise
  • Refined GitHub
  • Phabricator
  • Phabricator integration
  • Bitbucket
  • Bitbucket integration

• Test on different browsers (if applicable)

  • Chrome
  • Firefox
  • Safari

• Add change log message to client/browser/CHANGELOG.md, under "Unreleased" section. (if applicable)

Test plan