➕(app-impress) add @blocknote/mantine

The new version of blocknote dissociates
the ux from the @blocknote/react package.
It is now in a separate package @blocknote/mantine.
We add this package to the impress app.
We adapt the toolbar to fit the new package.

src/frontend/apps/impress/package.json

@@ -16,6 +16,7 @@
   },
   "dependencies": {
     "@blocknote/core": "0.13.2",
+    "@blocknote/mantine": "0.13.2",
     "@blocknote/react": "0.13.2",
     "@openfun/cunningham-react": "2.9.0",
     "@tanstack/react-query": "5.36.0",