Rebuild the theme assets with Bootstrap 5.3.9

Problem/Motivation

The theme requires "bootstrap": "^5.0.0" and commits its compiled css/ and js/, so releases ship assets built from an older Bootstrap.

Bootstrap 5.3.9 (11 November 2026) adds the .modal-xxl size, accessibility fixes and a fix for literal text in tooltips and popovers, with no breaking changes.

Steps to reproduce

1. Check out 5.0.x or 4.1.x.
2. Open package.json: bootstrap is ^5.0.0, and the committed css/ and js/ were built before 5.3.9.

Proposed resolution

• Raise the floor to "bootstrap": "^5.3.9" in package.json.
• Rebuild css/ and js/ from a clean install (Node.js 24 or later, Yarn 4.18.1, no lockfile).
• This needs an MR on each supported branch: 5.0.x and 4.1.x.
• MRs: !258 on 5.0.x and !259 on 4.1.x. Both also pin stylelint-prettier to 5.0.3, as Horizon Aid did in #3636458: Theme build fails and hangs without the lockfile: pin bootstrap-borders-utilities 1.0.2 and stylelint-prettier 5.0.3, so the theme build finishes.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Developer Documentation
• ➖ User Guide Documentation
• ❌ Reviewed by human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Release notes snippet
• ✅ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Rebuilt the compiled CSS and JS with Bootstrap 5.3.9, and raised the Bootstrap requirement to ^5.3.9.

AI-Generated: Yes