android: scan the web recipe card's QR and import the look it points at

The landing page's QR card now carries a link to `GET
/api/photos/:id/preset.recipe`, so the app can pick a look up off a
screen instead of a file. PRESETS gains a SCAN QR chip next to IMPORT:
the code is decoded, checked against the preset route, fetched, and
stored through the same `storeImportedRecipe` path IMPORT already uses,
so an imported look and a scanned look land in one place.

Scanning runs on `expo-camera`, not the viewfinder's vision-camera:
v5's object output is iOS-only, Android's `createObjectOutput` throws.
That is a second camera library in the app, so the sheet pauses the
viewfinder (`paused` prop) — CameraX will not let two clients hold one
lens. A native rebuild is required for the new module.

The code holds a URL, never the recipe, so anything that is not the
preset route is refused by name rather than silently dropped. On a
release APK the plain-HTTP link is blocked by Android's cleartext
policy; debug builds have it on. Noted in 7_SCAN_QR.md along with the
web side, the regex, and the paused-viewfinder rule.

app.json

@@ -49,6 +49,12 @@
      "isAccessMediaLocationEnabled": true
     }
    ],
+   [
+    "expo-camera",
+    {
+     "cameraPermission": "Allow RecipesCam to scan a recipe QR code."
+    }
+   ],
    "expo-sharing"
   ],
   "extra": {