Extend the theme editor: typography, button/link colours, custom CSS, more presets

Grew /admin/theme from colours-only to a full theme editor, all applied
live via website_settings + ThemeVars:
- Typography: body font (10 web-safe + Google options; Google fonts load
  via an injected <link>) and H1/H2/H3 sizes (globals.css now reads
  --size-heading-* vars).
- Buttons & links: secondary/danger button colours + link/link-hover.
- Custom CSS: a raw textarea injected after the theme variables (staff-
  trusted), for anything the controls don't cover.
- Presets: 6 → 13 (added Galaxy, Royal, Cyberpunk, Neon, Coffee, Arctic,
  Christmas). ThemeVars now injects all the new vars + the font link.

Verified live (prod, amx_test): saved font=mono / H1=44px / custom CSS →
the public home reflected --font-family "Courier New", --size-heading-h1
44px and the injected rule; reverted the test settings. tsc 0,
vitest 49/49, next build 0.

src/app/globals.css

@@ -205,16 +205,16 @@ body {
 
 /* ── App component classes (used across the converted pages) ── */
 h1 {
-  font-size: 30px;
+  font-size: var(--size-heading-h1, 30px);
   font-weight: 800;
   margin: 0.2em 0 0.5em;
 }
 h2 {
-  font-size: 22px;
+  font-size: var(--size-heading-h2, 22px);
   font-weight: 700;
 }
 h3 {
-  font-size: 18px;
+  font-size: var(--size-heading-h3, 18px);
   font-weight: 700;
 }
 a {