fix(onboarding): hide search results until typed, trim gallery grid to 2 rows

- search demo results were greyed-but-visible while the query was still
  typing, as if the app pre-knew the search; they're now invisible (space
  reserved, no layout jump) and fade in only once typing completes
- gallery preview dropped from 3 rows to 2 so the 'Click any tile' explainer
  text is visible without scrolling

src/components/onboarding/StepGalleryPreview.tsx

@@ -3,19 +3,16 @@ import { FakeTile, ReplayButton } from "./fakes";
 
 const REVEAL_MS = 280;
 
+// Two rows (cols-4) keeps the explainer text visible without scrolling.
 const TILE_PROPS: { favorite?: boolean; rating?: number; duration?: string }[] = [
   {},
   { favorite: true },
   {},
   { duration: "1:24" },
   { rating: 5 },
-  {},
   { favorite: true, rating: 3 },
-  {},
   { duration: "0:09" },
   {},
-  { rating: 4 },
-  {},
 ];
 
 const TILE_COUNT = TILE_PROPS.length;