Ban px from stylesheets: every length rem/em off the responsive root

Only the :root font-size definition itself stays absolute - it is the
base everything derives from. Embed-card inline styles in the editor
follow the same rule; already-stored embeds keep their old px inline
styles until re-edited.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

prosekit-editor.js

@@ -129,15 +129,15 @@ function defineGiteaRepoEmbed() {
           'data-description': description,
           'data-url': url,
           style:
-            'display:block;border:1px solid #ddd;border-radius:8px;' +
-            'padding:12px 16px;margin:8px 0;font-family:inherit;',
+            'display:block;border:0.06em solid #ddd;border-radius:0.5em;' +
+            'padding:0.75em 1em;margin:0.5em 0;font-family:inherit;',
         },
         [
           'a',
           { href: url, style: 'font-weight:600;text-decoration:none;color:inherit;' },
           `${owner}/${repo}`,
         ],
-        ['div', { style: 'opacity:0.7;font-size:0.9em;margin-top:4px;' }, description],
+        ['div', { style: 'opacity:0.7;font-size:0.9rem;margin-top:0.25em;' }, description],
       ]
     },
   })