a11y: add h1 headings to all pages (Redesign Phase B)

Every page now has exactly one <h1> per spec §2.3.
Six pages use sr-only <h1>; notes uses visible <h1> in toolbar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/pages/shopping.js

@@ -553,6 +553,7 @@ export async function render(container, { user }) {
 
   container.innerHTML = `
     <div class="shopping-page">
+      <h1 class="sr-only">Einkaufslisten</h1>
       <div class="list-tabs-bar" id="list-tabs-bar"></div>
       <div id="list-content" style="flex:1;display:flex;flex-direction:column;overflow:hidden"></div>
       <button class="page-fab" id="fab-new-item" aria-label="Artikel hinzufügen">