feat: add page templates for profile, settings, invoice, pricing, FAQ, errors

Adds eight static page templates that have been requested repeatedly --
v3 had most of these and they weren't ported in the v4 rewrite (#6011):

- pages/profile.html        avatar header, tabbed Activity / Timeline /
                            Settings panes, about card with skills
- pages/settings.html       left-rail pills for Account, Notifications,
                            Security, Billing, Danger Zone
- pages/invoice.html        print-ready invoice with totals math
                            computed in frontmatter, .d-print-none on
                            the action toolbar so the printed output is
                            clean
- pages/pricing.html        3-tier cards with billing toggle and a
                            feature comparison table
- pages/faq.html            data-driven accordions across three topic
                            groups, with a search bar header
- pages/404.html            full-screen centered, search + back-to-home
                            CTAs, no sidebar
- pages/500.html            full-screen centered, back-to-home +
                            contact-support CTAs
- pages/maintenance.html    full-screen centered with ETA card

All pages use pure Bootstrap 5 utility classes -- no custom CSS, no
external dependencies. Sidenav entries land in a later commit.