web: filter the straighten draw so a rotated edge stops staircasing

The FRAME tab's fine rotation drew the photo through canvas.rotate() +
canvas.scale() with a plain drawImage, which CanvasKit samples with
nearest: the edge landed on the same pixel in every row, so a rotated
edge came out as 1px steps every 1/tan(angle) rows. Measured on the 30deg
export of a hard black/white edge: 42.3% of rows repeated the previous
row's edge position, the step across the edge was 252.9 of 255, and there
were no intermediate pixels at all.

Only the *Options/*Cubic call shapes take a sampling option, and
drawImageRectOptions exists in RN Skia too, so the shared renderer can use
it unchanged. The same export now moves the edge in every row (0.2% of
rows repeat, 0.35 intermediate pixels per row) and its edge step drops to
222. Cost: the filtered draw takes 0.35s against 0.24s for the 1600px
preview copy and 2.0s against 1.4s for a 12MP photo, once per render.

Preview and export share the function, so both change together.

docker/frontend/shared/utils/skiaImage.ts

@@ -56,7 +56,20 @@ export function rotateSkImageBy(image: SkImage | null, degrees: number): SkImage
     canvas.translate(w / 2, h / 2);
     canvas.rotate(degrees, 0, 0);
     canvas.scale(k, k);
-    canvas.drawImage(image, -w / 2, -h / 2);
+    // A plain drawImage through a rotation is nearest-neighbour on CanvasKit: the
+    // edge lands on the same pixel in every row and the result staircases
+    // (measured: residual std of the edge position 0.289 vs 0.048 filtered, see
+    // straighten-sampling-probe.cjs). Only the *Options/*Cubic call shapes take a
+    // sampling option, and `drawImageRectOptions` exists in RN Skia too, so the
+    // shared renderer stays portable.
+    canvas.drawImageRectOptions(
+      image,
+      Skia.XYWHRect(0, 0, w, h),
+      Skia.XYWHRect(-w / 2, -h / 2, w, h),
+      Skia.FilterMode.Linear,
+      Skia.MipmapMode.None,
+      null
+    );
     return surface.makeImageSnapshot() ?? image;
   } finally {
     surface.dispose(); // see rotateSkImage90