Header flashes and reflows on every page navigation #259

Symptom. Switching pages reflows the header: gated links (Sessions, Reports, Tokens, Admin) are hidden in the markup and shown only after /api/me answers, the logo is revalidated on each load, and the nav differs from page to page (Credentials is missing on some), so the header jumps on first paint and admin-only controls pop in afterwards.

Proposed fix. Serve each page in five variants, one per role level, differing only in a data-role-level attribute on <html>; optional auth on the page routes so a logged-in request gets its variant on first paint, ETag per variant; the logo joins the content-addressed assets. Every page carries the same eight links with data-min-role on the gated ones and rustguac.css shows them from the attribute; API-key users set it from /api/me. Admin-only controls follow the same gate.

Fixed by feat/role-baked-pages-unified-nav. Part of the #254 series.