design: Desktop-UI modernisieren (Sidebar, Typografie, Shadows, Buttons)

- Sidebar: Logo mit Gradient, kompaktere Nav-Items (40px statt 44px),
  kleinere Icons (18px), 2px Gap, bessere Hover/Active-Zustände
- tokens.css: feinere Schatten, Inter im Font-Stack, Sidebar-Breite 256px
- layout.css: Desktop-Content mehr Luft (40px padding), Headings mit
  letter-spacing, Primary-Buttons mit subtiler Elevation
- router.js: Logo-Span für CSS-Gradient-Clip

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

public/router.js

@@ -138,7 +138,7 @@ async function renderPage(route) {
 function renderAppShell(container) {
   container.innerHTML = `
     <nav class="nav-sidebar" aria-label="Hauptnavigation">
-      <div class="nav-sidebar__logo">Oikos</div>
+      <div class="nav-sidebar__logo"><span>Oikos</span></div>
       <div class="nav-sidebar__items" role="list">
         ${navItems().map(navItemHtml).join('')}
       </div>