feat: stamp 300dpi metadata on camera-captured exports

Skia JPEG re-encode drops original header (72dpi camera default). New
src/utils/jpegDpi.ts rewrites JFIF APP0 density + EXIF X/YResolution
(endian-aware) or inserts minimal JFIF when neither exists; applied via
processAndExportPhoto options.dpi in the camera-capture branch only.
Library-mode exports untouched. Pixels never change - metadata only.

src/utils/exportEngine.ts

@@ -6,12 +6,18 @@ import { Recipe, GPSInfo, FrameId } from '../types';
 import { getSkiaColorMatrix } from './colorUtils';
 import { drawFrameOnCanvas } from './frameUtils';
 import { formatCoordinate } from './locationUtils';
+import { base64ToBytes, bytesToBase64, patchJpegDpi } from './jpegDpi';
 
 export interface ExportResult {
   uri: string;
   savedToLibrary: boolean;
 }
 
+export interface ExportOptions {
+  // DPI to stamp into the JPEG header (metadata only — pixel size never changes).
+  dpi?: number;
+}
+
 // Fixed filename: every export overwrites the previous one, so temp files
 // never accumulate in the cache directory.
 const EXPORT_FILE = `${FileSystem.cacheDirectory}camrecipe_pro_last_export.jpg`;
@@ -21,7 +27,8 @@ export async function processAndExportPhoto(
   recipe: Recipe,
   frameId: FrameId,
   useGeotag: boolean,
-  gpsInfo: GPSInfo | null
+  gpsInfo: GPSInfo | null,
+  options?: ExportOptions
 ): Promise<ExportResult | null> {
   try {
     // 1. Read source image file into Skia
@@ -181,13 +188,19 @@ export async function processAndExportPhoto(
 
     // 9. Snapshot & Encode to JPEG (base64 string, no SkData round-trip)
     const resultImage = surface.makeImageSnapshot();
-    const base64 = resultImage.encodeToBase64(ImageFormat.JPEG, 95);
+    let base64 = resultImage.encodeToBase64(ImageFormat.JPEG, 95);
 
     if (!base64) {
       console.error('Failed to encode image to JPEG');
       return null;
     }
 
+    // 9b. Patch DPI metadata (JFIF density / EXIF resolution) when requested
+    if (options?.dpi && options.dpi > 0) {
+      const patched = patchJpegDpi(base64ToBytes(base64), options.dpi);
+      base64 = bytesToBase64(patched);
+    }
+
     // 10. Write binary bytes to temporary local file (overwrites previous export)
     await FileSystem.writeAsStringAsync(EXPORT_FILE, base64, {
       encoding: FileSystem.EncodingType.Base64,