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/contacts.js

@@ -43,6 +43,7 @@ export async function render(container, { user }) {
   _container = container;
   container.innerHTML = `
     <div class="contacts-page">
+      <h1 class="sr-only">Kontakte</h1>
       <div class="contacts-toolbar">
         <div class="contacts-toolbar__search">
           <i data-lucide="search" class="contacts-toolbar__search-icon"></i>