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",

paginator.js

@@ -220,6 +220,8 @@ const makeMarginals = (length, part) => Array.from({ length }, () => {
 })
 
 const setStylesImportant = (el, styles) => {
+    // some generated section documents have no <body>/<html>
+    if (!el) return
     const { style } = el
     for (const [k, v] of Object.entries(styles)) style.setProperty(k, v, 'important')
 }
@@ -359,6 +361,8 @@ class View {
         const { width, height, margin } = this.#layout
         const vertical = this.#vertical
         const doc = this.document
+        // the iframe document may not be loaded yet on early resize renders
+        if (!doc?.body) return
         for (const el of doc.body.querySelectorAll('img, svg, video')) {
             // preserve max size if they are already set
             const { maxHeight, maxWidth } = doc.defaultView.getComputedStyle(el)