Fix incorrect facet offsets with non-ASCII characters

src/conversion.ts

···
   offset ??= 0;
   parents ??= [];
   let blocks: PubLeafletPagesLinearDocument.Block[] = [];
+  const encoder = new TextEncoder("utf-8");
 
   for (const content of children) {
     if (content.type == "text") {
···
             index: {
               $type: "pub.leaflet.richtext.facet#byteSlice",
               byteStart: offset,
-              byteEnd: offset + content.value.length,
+              byteEnd: offset + encoder.encode(content.value).length,
             },
           },
           ...facets,
         ];
 
       text += content.value;
-      offset += content.value.length;
+      offset += encoder.encode(content.value).length;
     } else if (content.type == "break") {
       blocks.push({
         $type: "pub.leaflet.pages.linearDocument#block",
···
           index: {
             $type: "pub.leaflet.richtext.facet#byteSlice",
             byteStart: offset,
-            byteEnd: offset + content.value.length,
+            byteEnd: offset + encoder.encode(content.value).length,
           },
           features: [{ $type: "pub.leaflet.richtext.facet#code" }],
         },
         ...facets,
       ];
       text += content.value;
-      offset += content.value.length;
+      offset += encoder.encode(content.value).length;
     } else if (content.type == "image") {
       if (text != "") {
         blocks.push({