HOLD: Add page-header design reference (styleguide)

[maebeale]

What is the goal of this PR and why is this important?

• Page headings across the app have drifted into ~5 inconsistent variants (h1 vs h2, semibold/bold, text-2xl/3xl, centered vs left, ad-hoc back-links and breadcrumb rows) spread over 500+ templates.
• Before building shared header partials and sweeping the site, we need to agree on the target look-and-feel. This PR adds an internal design-reference page so those decisions can be made visually, against real design tokens, rather than in the abstract.

How did you approach the change?

• Added GET /styleguide/headers (StyleguideController#headers) — logged-in only, no model data, authorization skipped.
• Built app/views/styleguide/headers.html.erb plus two demo partials (_demo, _utility_row) that render every candidate treatment with sample content using the real DomainTheme color tokens.
• The page is organized as a decision surface:
  • Anatomy + rules — one shared utility row (back-link left → defaults to admin_path on admin pages; subnav right), actions in the title row.
  • Families — list (A), topper (B), centered hero (C), and the proposed charcoal base + thin model-color stripe (J) that unifies admin chrome while marking individual models.
  • Questions — alignment/centered-vs-left (E), event registration admin-vs-public (F), band-everywhere combo (G), more subnavs per-record & per-area (H), and rich gradients for public contribution forms (I).
• Organizing principle surfaced by the exercise: rich = public-facing, quiet = admin (gradients/centered heroes signal outward pages; charcoal/flat signal internal tools).

Anything else to add?

• This is a reference/spec only — no existing pages are changed yet. Once the variants are chosen, the follow-up builds shared/_page_header + shared/_subnav and converts pages in batches.
• Per-model stripe/gradient colors are dynamic, so they'll need adding to the Tailwind safelist alongside the existing DomainTheme colors when the real partials land.
• Open decisions captured on the page: background alignment, Style A for the admin registration form, charcoal+stripe vs light band, subnav standard (D1), subnav expansion, and gradient approach for public forms.

[maebeale]

🤖 From Claude: Auth is skipped (not authorize!) because this is a static, data-free internal reference with no records to authorize against. Still behind authenticate_user!, so logged-in only.

[maebeale]

🤖 From Claude: Reference/spec route only — meant to be removed (or moved behind an admin constraint) once the header system is chosen and the real shared partials land.