fix: show share button on all content pages instead of only TOC-matched pages

The v-if condition used shell.currentPage.length > 0, which requires the
current URL to match a TOC entry. With placeholder content, this array is
often empty even on valid content pages. Changed to use shell.isContentPage
which correctly identifies content pages regardless of TOC matching.

Author: BenjaminMichaelis

EssentialCSharp.Web/src/components/PageShareAction.vue

@@ -5,7 +5,7 @@ const shell = inject("shell");
 </script>
 
 <template>
-    <div v-if="shell.currentPage.length > 0" class="page-share-action">
+    <div v-if="shell.isContentPage" class="page-share-action">
         <div class="turn-page-tooltip">
             <button
                 id="share-btn"