Move the CROP band, not the picture

Library CROP now resizes the band in photo fractions, the way FREE always
did, instead of panning a re-scaled photo under a screen-sized rectangle:
what the band encloses is the export, with no zoom transform left to undo
and no aspect that only holds on a square photo. The band is laid out
below the top strip, whose height TopBar now reports, so it can never
peek under the header. The grain hashes a cell of width/1080 instead of
the raw pixel grid, so a 4000px export is no longer ~4x finer than the
preview that tuned it.

src/components/TopBar.tsx

@@ -21,6 +21,9 @@ interface TopBarProps {
   // drawn: the icon turns amber while there is a step to go back to.
   onUndo: () => void;
   undoDepth: number;
+  // Height of the floating top strip (its own onLayout): App keeps the library
+  // photo — and with it the ratio band — out from under it.
+  onHeight?: (height: number) => void;
 }
 
 export default function TopBar({
@@ -32,6 +35,7 @@ export default function TopBar({
   onExportShare,
   onUndo,
   undoDepth,
+  onHeight,
 }: TopBarProps) {
   const insets = useSafeAreaInsets();
   const [modalVisible, setModalVisible] = useState(false);
@@ -74,6 +78,7 @@ export default function TopBar({
     <>
       <View
         style={{ paddingTop: insets.top + 6 }}
+        onLayout={(e) => onHeight?.(e.nativeEvent.layout.height)}
         className="absolute top-0 left-0 right-0 z-20 flex-row items-start justify-between px-4"
       >
         <View className="max-w-[62%] rounded-xl border border-zinc-800/80 bg-black/45 px-3 py-2">