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/calendar.js

@@ -184,6 +184,7 @@ function renderToolbar() {
   if (!bar) return;
 
   bar.innerHTML = `
+    <h1 class="sr-only">Kalender</h1>
     <div class="cal-toolbar__nav">
       <button class="btn btn--icon" id="cal-prev" aria-label="Zurück">
         <i data-lucide="chevron-left"></i>