fix: fix nav scrolling and card layout on mobile

- App.jsx: use flex column layout so BottomNav stays at bottom and
  never scrolls away; Feed gets a proper definite height for scroll-snap
- Feed.css: feed-slot uses min-height so long cards can grow, align top
- PairCards.css: cap image at 40dvh so card always fits on screen

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

src/components/PairCards.css

@@ -83,6 +83,8 @@
   position: relative;
   width: 100%;
   aspect-ratio: 1 / 1;
+  /* Bild nie höher als ~40% des Viewport, damit die Karte komplett sichtbar ist */
+  max-height: 40dvh;
   overflow: hidden;
   cursor: default;
   background: #1a1a1a;