fix: admin panel light/dark theme support, improved contrast, and backup confirmation

a) Admin panel now respects the site's light/dark theme toggle via CSS
   custom properties that respond to [data-theme='light'] on <html>.
   All hardcoded dark colors replaced with semantic CSS variables.

b) Improved contrast in dark mode: secondary text changed from #666/#555
   to #a0a0a0, muted text from #555 to #707070. All text is now clearly
   readable against dark backgrounds.

c) Backup system was already implemented in yaml-service.ts (creates
   timestamped backups in content/.backups/ before each save, keeps last
   10 versions). Now the UI shows a confirmation message that a backup
   was created after each successful save.

app/admin/components/PageBuilder.tsx

@@ -226,9 +226,10 @@ export default function PageBuilder() {
       });
 
       if (res.ok) {
+        const data = await res.json();
         setDirty(false);
-        setSaveMessage('Saved successfully!');
-        setTimeout(() => setSaveMessage(''), 3000);
+        setSaveMessage(data.message || 'Saved successfully!');
+        setTimeout(() => setSaveMessage(''), 5000);
       } else {
         const err = await res.json();
         setSaveMessage(`Error: ${err.error}`);