fix: restore TextSettings functionality after store refactoring

Fixed TextSettings fields not updating the stylesheet and preview after
the store refactoring that made content a computed property.

- Add missing font watcher in TextSettings.vue
- Update useCssUpdater.js to use store.replaceBlock() instead of
  writing to readonly store.content
- Update createRule() to append to store.customCss instead of store.content

All TextSettings fields (font, size, margins, padding, alignment) now
correctly update the stylesheet and preview.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/components/editor/TextSettings.vue

@@ -452,6 +452,11 @@ const updateMarginInnerUnit = (unit) => {
 };
 
 // Watchers for body styles
+watch(font, (val) => {
+  if (isUpdatingFromStore) return;
+  updateStyle('body', 'font-family', `"${val}"`);
+});
+
 watch(italic, (val) => {
   if (isUpdatingFromStore) return;
   updateStyle('body', 'font-style', val ? 'italic' : 'normal');