Upgrade the website from Bootstrap 3.2.0 to Bootstrap 5.3.8

Bootstrap 3.2.0 reached end of life years ago and required jQuery, the last external script on the site. Bootstrap 5 is self-hosted like before and needs no jQuery at all, so the vendored copy added for the CSP fix is gone again. The navbar, dropdowns, and mobile toggle use Bootstrap 5 markup, the index hero replaces the removed jumbotron component, and the prose pages drop a column-less row wrapper that Bootstrap 5’s gutters turned into misaligned code blocks and lists. The site adopts Bootstrap 5’s native fonts, colors, and spacing; the only copy change is the footer year moving to 2026. All pages, dropdowns, and the mobile menu verified locally, with visible text diffed against the live site to confirm nothing else changed.