Give the homepage teaser a rich text editor

Replace the plain textarea for the homepage_teaser setting with the shared
TipTap rich-text editor, and render the teaser as sanitized HTML in the
portal hero's default layout.

- SettingsAdmin: teaser field now uses RichTextEditor (lazy-loaded, code-split
  like PostEditor); rich fields render in a <div> wrapper instead of <label>.
- HeroElement: text-block lines flagged `html` render sanitized HTML.
- heroLayout: the default-layout teaser line is now an HTML line.
- admin.controller: sanitize homepage_teaser against the body allowlist on save.
- theme.css: collapse the teaser's nested block margins in the hero.

Closes #48

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/lib/heroLayout.js

@@ -64,7 +64,7 @@ export function defaultLayout(teaser) {
             { text: 'Private shard project', tag: 'span', fontSize: '0.74rem', color: '#c2d2e6', weight: 700, letterSpacing: '0.22em', transform: 'uppercase', font: 'sans' },
             { text: 'UOMysticmoon', tag: 'h1', fontSize: 'clamp(3rem,8.5vw,5.75rem)', color: 'var(--head)', weight: 600, letterSpacing: '0.02em', lineHeight: 1, font: 'display', marginTop: 14 },
             { text: 'A private Ultima Online world in progress', tag: 'p', fontSize: '1.32rem', color: '#dbe2ea', italic: true, marginTop: 22 },
-            { text: teaser, tag: 'p', fontSize: '1.06rem', color: '#c4cdd8', maxWidth: 600, marginTop: 22 },
+            { text: teaser, tag: 'div', html: true, fontSize: '1.06rem', color: '#c4cdd8', maxWidth: 600, marginTop: 22 },
           ],
         },
       },