refactor(auth): unify messenger bot link flow across login/profile/notifications

app/composables/useMessengerBotLink.ts

@@ -0,0 +1,10 @@
+export function buildMessengerBotStartUrl(baseUrl: string, email: string) {
+  const normalizedEmail = email.trim().toLowerCase();
+  if (!baseUrl || !normalizedEmail) {
+    return '';
+  }
+
+  const payload = encodeURIComponent(`login:${normalizedEmail}`);
+  const separator = baseUrl.includes('?') ? '&' : '?';
+  return `${baseUrl}${separator}start=${payload}`;
+}