CI: verify gravity slider is present in built JS bundle

[Testtesttestestes]

Motivation

• Ensure the production build actually includes the Gravity slider code by failing CI early if the built bundle is missing the expected identifier.

Description

• Added a new step to the build job in .github/workflows/build-static-page.yml that finds dist/assets/index-*.js and greps for the string Gravity, exiting non-zero if no built JS bundle is found or the string is absent.
• The verification runs after npm run build and before preparing/uploading the release artifact.

Testing

• The workflow runs the automated steps npm ci, npm run lint (type check), and npm run build as part of CI configuration.
• The new verification step executes during CI and will cause the job to fail if no dist/assets/index-*.js file is present or if the Gravity string is not found.
• No execution logs are included in this diff, so CI run results are not shown here and should be observed in the repository's workflow run output.

---

Codex Task