feat: full-res screen sharpen on camera-captured exports

Lightroom-style 'Screen' output sharpen (unsharp mask, 3x3 convolution
via Skia ImageFilter.MakeMatrixConvolution, amount 0.5, kernel sums to
1) applied to final pixels before JPEG encode. Camera-capture branch
passes { dpi: 300, sharpen: true }; library-mode export untouched.
Export pixels were already full resolution - sharpen is a new final
GPU pass, no pixel-count change.

src/utils/exportEngine.ts

@@ -1,4 +1,4 @@
-import { Skia, ImageFormat, BlendMode, TileMode } from '@shopify/react-native-skia';
+import { Skia, ImageFormat, BlendMode, TileMode, type SkImage } from '@shopify/react-native-skia';
 import * as FileSystem from 'expo-file-system/legacy';
 import * as MediaLibrary from 'expo-media-library/legacy';
 import { Asset } from 'expo-asset';
@@ -16,6 +16,34 @@ export interface ExportResult {
 export interface ExportOptions {
   // DPI to stamp into the JPEG header (metadata only — pixel size never changes).
   dpi?: number;
+  // Apply a subtle Lightroom-style "Screen" output sharpen (unsharp mask) to the
+  // final pixels before encoding. Pixels already export at full resolution.
+  sharpen?: boolean;
+}
+
+// Lightroom-style "Screen" output sharpen: subtle unsharp mask. A 3x3
+// convolution whose kernel sums to 1 (brightness preserved), amount ~0.5.
+// ponytail: fixed subtle preset tuned for full-res desktop/screen output; turn
+// into an amount/radius UI control if users ask for adjustable strength.
+function screenSharpenImage(image: SkImage, amount = 0.5): SkImage {
+  const w = image.width();
+  const h = image.height();
+  const center = 1 + 4 * amount;
+  const filter = Skia.ImageFilter.MakeMatrixConvolution(
+    3, 3,
+    [0, -amount, 0, -amount, center, -amount, 0, -amount, 0],
+    1, 0, 1, 1, TileMode.Clamp, false, null, null
+  );
+  const surf = Skia.Surface.Make(w, h);
+  if (!surf) return image; // fall back to unsharpened on allocation failure
+  const paint = Skia.Paint();
+  paint.setImageFilter(filter);
+  const c = surf.getCanvas();
+  c.clear(Skia.Color('transparent'));
+  c.drawImage(image, 0, 0, paint);
+  const sharpened = surf.makeImageSnapshot();
+  if (!sharpened) return image;
+  return sharpened;
 }
 
 // Fixed filename: every export overwrites the previous one, so temp files
@@ -187,7 +215,10 @@ export async function processAndExportPhoto(
     }
 
     // 9. Snapshot & Encode to JPEG (base64 string, no SkData round-trip)
-    const resultImage = surface.makeImageSnapshot();
+    let resultImage = surface.makeImageSnapshot();
+    if (options?.sharpen) {
+      resultImage = screenSharpenImage(resultImage);
+    }
     let base64 = resultImage.encodeToBase64(ImageFormat.JPEG, 95);
 
     if (!base64) {