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/dashboard.js

@@ -438,6 +438,7 @@ export async function render(container, { user }) {
 
   container.innerHTML = `
     <div class="dashboard">
+      <h1 class="sr-only">Übersicht</h1>
       <div class="dashboard__grid">
         ${renderGreeting(user, stats)}
         ${renderWeatherWidget(weather)}