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/__tests__/lib/instagramShare.test.ts

@@ -0,0 +1,73 @@
+import { Linking } from "react-native";
+
+const mockShareSingle = jest.fn();
+const mockIsPackageInstalled = jest.fn();
+
+jest.mock("react-native-share", () => ({
+    __esModule: true,
+    default: {
+        shareSingle: (...args: unknown[]) => mockShareSingle(...args),
+        isPackageInstalled: (...args: unknown[]) => mockIsPackageInstalled(...args),
+    },
+    Social: { InstagramStories: "instagramstories" },
+}));
+
+function loadShareToInstagramStories() {
+    let shareToInstagramStories: (uri: string) => Promise<boolean>;
+    jest.isolateModules(() => {
+        shareToInstagramStories = require("../../lib/instagramShare").shareToInstagramStories;
+    });
+    return shareToInstagramStories!;
+}
+
+describe("shareToInstagramStories", () => {
+    const originalAppId = process.env.EXPO_PUBLIC_FACEBOOK_APP_ID;
+
+    afterEach(() => {
+        if (originalAppId === undefined) {
+            delete process.env.EXPO_PUBLIC_FACEBOOK_APP_ID;
+        } else {
+            process.env.EXPO_PUBLIC_FACEBOOK_APP_ID = originalAppId;
+        }
+    });
+
+    it("returns false without opening Instagram when no Facebook App ID is configured", async () => {
+        delete process.env.EXPO_PUBLIC_FACEBOOK_APP_ID;
+        const share = loadShareToInstagramStories();
+
+        await expect(share("file:///tmp/card.png")).resolves.toBe(false);
+        expect(mockShareSingle).not.toHaveBeenCalled();
+    });
+
+    it("returns false when Instagram is not installed", async () => {
+        process.env.EXPO_PUBLIC_FACEBOOK_APP_ID = "123456";
+        jest.spyOn(Linking, "canOpenURL").mockResolvedValue(false);
+        const share = loadShareToInstagramStories();
+
+        await expect(share("file:///tmp/card.png")).resolves.toBe(false);
+        expect(mockShareSingle).not.toHaveBeenCalled();
+    });
+
+    it("shares the captured image as the story background when Instagram is available", async () => {
+        process.env.EXPO_PUBLIC_FACEBOOK_APP_ID = "123456";
+        jest.spyOn(Linking, "canOpenURL").mockResolvedValue(true);
+        mockShareSingle.mockResolvedValue({ success: true });
+        const share = loadShareToInstagramStories();
+
+        await expect(share("file:///tmp/card.png")).resolves.toBe(true);
+        expect(mockShareSingle).toHaveBeenCalledWith({
+            social: "instagramstories",
+            appId: "123456",
+            backgroundImage: "file:///tmp/card.png",
+        });
+    });
+
+    it("returns false when the native share fails", async () => {
+        process.env.EXPO_PUBLIC_FACEBOOK_APP_ID = "123456";
+        jest.spyOn(Linking, "canOpenURL").mockResolvedValue(true);
+        mockShareSingle.mockRejectedValue(new Error("share failed"));
+        const share = loadShareToInstagramStories();
+
+        await expect(share("file:///tmp/card.png")).resolves.toBe(false);
+    });
+});