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/notes.js

@@ -47,7 +47,7 @@ export async function render(container, { user }) {
   container.innerHTML = `
     <div class="notes-page">
       <div class="notes-toolbar">
-        <span class="notes-toolbar__title">Pinnwand</span>
+        <h1 class="notes-toolbar__title">Pinnwand</h1>
         <button class="btn btn--primary" id="notes-add-btn">
           <i data-lucide="plus" style="width:16px;height:16px;margin-right:4px;"></i>
           Neue Notiz