a11y: Skip-Link für Keyboard-Navigation (Redesign Phase D)

- Skip-Link "Zum Inhalt springen" als erstes Element im App-Shell
- .sr-only:focus-visible CSS: sichtbar bei Tab-Focus, fixed top-left

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

public/router.js

@@ -145,6 +145,7 @@ async function renderPage(route) {
  */
 function renderAppShell(container) {
   container.innerHTML = `
+    <a href="#page-content" class="sr-only">Zum Inhalt springen</a>
     <nav class="nav-sidebar" aria-label="Hauptnavigation">
       <div class="nav-sidebar__logo"><span>Oikos</span></div>
       <div class="nav-sidebar__items" role="list">