fix(pwa): fix UI overlap, scroll bleed and wrong nav height on iOS

Three root causes fixed:

1. Double safe-area padding: pwa.css set padding-top/bottom on body
   globally, but page containers already account for safe-area-inset
   in their height calculations. Removed body vertical padding (kept
   only in standalone media query for padding-top).

2. Wrong nav token: all page containers used --nav-height-mobile (56px)
   instead of --nav-bottom-height (68px = 56px scroll + 12px dots),
   causing 12px of content to render behind the bottom nav.

3. Scroll bleed: fixed-height page containers lacked overflow:hidden,
   allowing scroll events to propagate to the body. Added
   overscroll-behavior-y:contain on app-content globally.

Fixes #16

public/styles/pwa.css

@@ -15,10 +15,11 @@ html, body {
   -webkit-tap-highlight-color: transparent;
 }
 
-/* ── Safe Area Insets (Notch, Dynamic Island, Gesture Bar) ── */
+/* ── Safe Area Insets (Notch, Dynamic Island, Gesture Bar) ──
+ * Nur horizontale Safe Areas auf body - vertikale werden von
+ * Standalone-Modus (padding-top) und Nav/Seiten (padding-bottom) gehandhabt.
+ * Kein body padding-top/bottom hier, sonst doppelt mit Seiten-Berechnungen. */
 body {
-  padding-top: env(safe-area-inset-top);
-  padding-bottom: env(safe-area-inset-bottom);
   padding-left: env(safe-area-inset-left);
   padding-right: env(safe-area-inset-right);
 }
@@ -61,4 +62,9 @@ nav,
   body {
     padding-top: env(safe-area-inset-top);
   }
+
+  /* Kein Scroll-Bleed - Content bleibt in seinem Container */
+  .app-content {
+    overscroll-behavior-y: contain;
+  }
 }