🐛(frontend) fix styles in doc editor heading

The headings in the doc editor were not
inheriting the correct style anymore due to a recent
upgrade of Blocknote.
This commit fixes the issue.

Author: AntoLC

src/frontend/apps/impress/src/features/docs/doc-editor/styles.tsx

@@ -123,13 +123,13 @@ export const cssEditor = css`
     .bn-inline-content {
     text-decoration: none;
   }
-  h1 {
+  .bn-default-styles h1 {
     font-size: 1.875rem;
   }
-  h2 {
+  .bn-default-styles h2 {
     font-size: 1.5rem;
   }
-  h3 {
+  .bn-default-styles h3 {
     font-size: 1.25rem;
   }
   a {