fix(pwa): disable pinch-to-zoom and block residual body scroll (#16)

- Added user-scalable=no, maximum-scale=1 to viewport meta tag to prevent
  pinch-to-zoom in standalone PWA mode
- Added overflow: hidden to html, body so any minimal content overflow
  cannot make the body scrollable (belt-and-suspenders alongside
  overscroll-behavior: none)
- Service worker cache bumped to v28/v27

public/styles/pwa.css

@@ -4,8 +4,12 @@
  * Abhängigkeiten: tokens.css, layout.css
  */
 
-/* ── Kein Rubber-Banding / Pull-to-Refresh des Browsers ── */
+/* ── Kein Rubber-Banding, kein Body-Scroll ──
+ * overflow: hidden verhindert, dass body scrollbar wird, falls ein
+ * Element minimal überläuft. overscroll-behavior: none deaktiviert
+ * Pull-to-Refresh und Rubber-Banding. */
 html, body {
+  overflow: hidden;
   overscroll-behavior: none;
 }