fix(paginator): tolerate not-yet-loaded section documents in early resize renders

The ResizeObserver can fire while the iframe document is still unset:
setImageSize dereferenced doc.body unconditionally and
setStylesImportant destructure-crashed on null, killing the render
loop — the view then never snapped or relocated again (observed in an
embedded Android WebView where the first observation lands pre-load).
Guard both paths.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "foliate-js",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "description": "Render e-books in the browser",
   "repository": {
     "type": "git",