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/App.jsx

@@ -29,8 +29,10 @@ function AppContent() {
   const PageComponent = PAGES[page] || Feed
 
   return (
-    <div style={{ minHeight: '100vh', background: '#F5F0E8', paddingBottom: '72px' }}>
-      <PageComponent />
+    <div style={{ flex: 1, display: 'flex', flexDirection: 'column', background: '#F5F0E8', overflow: 'hidden' }}>
+      <div style={{ flex: 1, overflow: 'hidden', minHeight: 0 }}>
+        <PageComponent />
+      </div>
       <BottomNav active={page} onNavigate={setPage} />
     </div>
   )