Add tappable cards, detail bottom sheet, swipe gestures, and favorites view

- Decision types, services (decisionService, listingDetailService), and index exports
- useDecisions hook with optimistic updates and Map-based state
- useListingDetail hook with session-level caching
- PhotoCarousel component using embla-carousel-react
- ListingDetail component with full property info, like/dislike buttons
- ListingDetailSheet using vaul Drawer (slide-up bottom sheet)
- SwipeablePropertyCard with @use-gesture/react and @react-spring/web
- SwipeReviewMode for mobile full-screen swipe review
- FavoritesView with virtualized liked listings and remove button
- App.tsx integration: decision state, client-side disliked filtering, detail sheet, swipe handlers
- ListView conditionally renders SwipeablePropertyCard when handlers provided
- StatsBar adds 'saved' view mode with heart icon
- Header adds liked count indicator
- New deps: vaul, embla-carousel-react, @use-gesture/react, @react-spring/web

frontend/src/components/MobileBottomSheet.tsx

@@ -25,7 +25,7 @@ export function MobileBottomSheet({
     onActiveListingChange,
     poiMetricSelection,
 }: MobileBottomSheetProps) {
-    const [snap, setSnap] = useState<string | number>("148px");
+    const [snap, setSnap] = useState<string | number | null>("148px");
     const [activeCardIndex, setActiveCardIndex] = useState(0);
 
     const features = listingData?.features ?? [];