fix(share): attach captured card image when sharing to Instagram Stories

Both share modals opened the instagram-stories:// scheme without ever
placing the captured PNG on the pasteboard, so Instagram opened an empty
story. Replace the manual scheme launch with react-native-share
shareSingle(INSTAGRAM_STORIES), which writes the image to UIPasteboard
under com.instagram.sharedSticker.backgroundImage on iOS and uses the
ADD_TO_STORY intent on Android. Gate the flow on the required Facebook
App ID (EXPO_PUBLIC_FACEBOOK_APP_ID) and Instagram being installed, with
fallback to the system share sheet, and remove the dead base64 reads.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package-lock.json

@@ -381,6 +381,7 @@
         "react-native-reanimated": "~4.1.1",
         "react-native-safe-area-context": "~5.6.0",
         "react-native-screens": "~4.16.0",
+        "react-native-share": "^12.3.1",
         "react-native-svg": "15.12.1",
         "react-native-url-polyfill": "^2.0.0",
         "react-native-view-shot": "^4.0.3",
@@ -25256,6 +25257,15 @@
         "react-native": "*"
       }
     },
+    "node_modules/react-native-share": {
+      "version": "12.3.1",
+      "resolved": "https://registry.npmjs.org/react-native-share/-/react-native-share-12.3.1.tgz",
+      "integrity": "sha512-mRVRie0qKbtj+jWqJ2POPkeSd8SxnMp6aazFZVmq8ZbkUGtQLENR/L58ky8zH4VUEF/WYRmdBiBHWHVtzOewtQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=16"
+      }
+    },
     "node_modules/react-native-svg": {
       "version": "15.12.1",
       "resolved": "https://registry.npmjs.org/react-native-svg/-/react-native-svg-15.12.1.tgz",