🐛(frontend) fix svg export

Last upgrade of Blocknote to 0.30.0 broke the SVG
export. The previewWidth can be undefined, which causes the
export to fail. This commit adds a fallback
width in case previewWidth is undefined.

src/frontend/apps/e2e/__tests__/app-impress/assets/test.svg

@@ -1,4 +1,9 @@
-<svg width="100" height="100" viewBox="0 0 100 100">
+<svg
+  width="100"
+  height="100"
+  viewBox="0 0 100 100"
+  xmlns="http://www.w3.org/2000/svg"
+>
   <circle cx="50" cy="30" r="20" fill="#3498db" />
   <polygon
     points="50,10 55,20 65,20 58,30 60,40 50,35 40,40 42,30 35,20 45,20"