feat: let the login page send the token back to the mobile app

src/ui/lib/app-return.ts

@@ -0,0 +1,61 @@
+/**
+ * Sign in for the Termix mobile app in a real browser. The app opens the
+ * login page with ?return_to=termix-mobile://..., and after any login method
+ * the page sends the token back to that address. Only the app's own scheme
+ * is accepted, so the value can never send a token to a website.
+ */
+
+const STORAGE_KEY = "termix-app-return";
+const PARAM = "return_to";
+const APP_RETURN = /^termix-mobile:\/\/[A-Za-z0-9._~/-]*$/;
+
+export function isAppReturn(value: unknown): value is string {
+  return typeof value === "string" && APP_RETURN.test(value);
+}
+
+/** Reads return_to from the address bar once and keeps it for this tab. */
+export function captureAppReturn(): string | null {
+  try {
+    const url = new URL(window.location.href);
+    const value = url.searchParams.get(PARAM);
+    if (value !== null) {
+      url.searchParams.delete(PARAM);
+      window.history.replaceState({}, document.title, url.toString());
+      if (isAppReturn(value)) sessionStorage.setItem(STORAGE_KEY, value);
+      else sessionStorage.removeItem(STORAGE_KEY);
+    }
+    return pendingAppReturn();
+  } catch {
+    return null;
+  }
+}
+
+export function pendingAppReturn(): string | null {
+  try {
+    const value = sessionStorage.getItem(STORAGE_KEY);
+    return isAppReturn(value) ? value : null;
+  } catch {
+    return null;
+  }
+}
+
+export function appReturnUrl(
+  base: string,
+  params: Record<string, string>,
+): string {
+  const query = new URLSearchParams(params).toString();
+  return query ? `${base}?${query}` : base;
+}
+
+/** Sends the result to the app. False when no app asked for it. */
+export function handOffToApp(params: Record<string, string>): boolean {
+  const base = pendingAppReturn();
+  if (!base) return false;
+  try {
+    sessionStorage.removeItem(STORAGE_KEY);
+  } catch {
+    // The redirect still goes out.
+  }
+  window.location.replace(appReturnUrl(base, params));
+  return true;
+}