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>

apps/mobile/src/lib/instagramShare.ts

@@ -0,0 +1,48 @@
+import { Linking, Platform } from "react-native";
+import Share, { Social } from "react-native-share";
+
+// Instagram requires a registered Facebook App ID for the Stories share flow
+// (https://developers.facebook.com/docs/instagram/sharing-to-stories/).
+const facebookAppId = process.env.EXPO_PUBLIC_FACEBOOK_APP_ID;
+
+const INSTAGRAM_ANDROID_PACKAGE = "com.instagram.android";
+
+async function isInstagramStoriesAvailable(): Promise<boolean> {
+    if (!facebookAppId) return false;
+
+    try {
+        if (Platform.OS === "ios") {
+            return await Linking.canOpenURL("instagram-stories://share");
+        }
+        if (Platform.OS === "android") {
+            const { isInstalled } = await Share.isPackageInstalled(INSTAGRAM_ANDROID_PACKAGE);
+            return isInstalled;
+        }
+        return false;
+    } catch {
+        return false;
+    }
+}
+
+/**
+ * Share a locally captured image (file:// URI) to Instagram Stories as the
+ * story background. Returns true when the share was handed off to Instagram,
+ * false when Instagram Stories sharing is unavailable (not installed, no
+ * Facebook App ID configured, or the user dismissed the native prompt) so
+ * callers can fall back to the system share sheet.
+ */
+export async function shareToInstagramStories(imageUri: string): Promise<boolean> {
+    if (!(await isInstagramStoriesAvailable())) return false;
+
+    try {
+        const result = await Share.shareSingle({
+            social: Social.InstagramStories,
+            appId: facebookAppId as string,
+            backgroundImage: imageUri,
+        });
+        return result.success !== false;
+    } catch (error) {
+        console.error("Instagram Stories share error:", error);
+        return false;
+    }
+}