Scope Bootstrap tweaks to teal root + Shiny modal

[AleKoure]

Pull Request

Fixes #1688

• Problem: Bootstrap var tweaks in inst/css/custom.css leaked into host apps.
• Tradeoff: We scope them to .teal-root to avoid host leakage, but Shiny modals render under #shiny-modal-wrapper (outside .teal-root).
• Fix: Apply the same rules to #shiny-modal so teal modals keep consistent styling while limiting impact to the host app (only the modal container).

Longer‑term: we may consider a cleaner way to target teal components and move these values into bs_theme() or more formal app tokens.

[github-actions]

🎉 Thank you for your contribution! Before this PR can be accepted, we require that you read and agree to our Contributor License Agreement.
You can digitally sign the CLA by posting a comment on this Pull Request in the format shown below. This agreement will apply to this PR as well as all future contributions on this repository.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}