Keep ratio bands at native sensor zoom

The band region was the ratio photo re-scaled to the screen width, so 3:2
(0.667) rendered taller than the 4:3 (== native 3:4) band and zoomed the
scene. Anchor the band on the contain-fitted native sensor box instead and
cut the ratio strip out of it at the same scale: 3:2 is now the identical
vertical view with a narrower horizontal crop, never a taller image.

src/components/Viewfinder.tsx

@@ -126,13 +126,21 @@ export default function Viewfinder({
     (selectedFrame === 'none' || selectedFrame === 'classic-white' || selectedFrame === 'cinematic');
   const region = useMemo(() => {
     if (!regionBand || ratioWH === null) return { x: 0, y: 0, w: vw, h: vh };
-    const h = vw / ratioWH;
-    if (h <= vh) {
-      return { x: 0, y: (vh - h) / 2, w: vw, h };
-    }
-    // Screen narrower than the ratio: fit by height, letterbox the sides.
-    const w = vh * ratioWH;
-    return { x: (vw - w) / 2, y: 0, w, h };
+    // The band is a same-zoom crop WINDOW over the full sensor view, not the
+    // photo re-scaled to the screen width: anchor on the native sensor box
+    // (upright 3:4, contain-fitted to the screen) and cut the ratio strip out
+    // of it at the same scale. 4:3 (== native) therefore fills the full sensor
+    // width at the reference zoom, and 3:2 is the identical vertical view with
+    // a narrower horizontal crop — switching ratio never zooms the scene, so a
+    // 3:2 band never shows a taller image than 4:3.
+    // ponytail: sensor assumed upright 3:4 (quality-mode captures); if a device
+    // negotiates another photo aspect, derive it from the session instead.
+    const nativeWH = 3 / 4;
+    const nativeH = Math.min(vh, vw / nativeWH);
+    const nativeW = nativeH * nativeWH;
+    const h = ratioWH <= nativeWH ? nativeH : nativeW / ratioWH;
+    const w = ratioWH * h;
+    return { x: (vw - w) / 2, y: (vh - h) / 2, w, h };
   }, [regionBand, ratioWH, vw, vh]);
   // Camera outputs: attach the DNG output only while RAW is on — CameraX
   // cannot bind two ImageCapture use cases at once, and App's session-error