feat: Add auth handoff mutation operations

Adds GraphQL operations for:
- CreateAuthHandoffToken: Get handoff token for mobile-to-web auth
- ExchangeAuthHandoffToken: Exchange token for Firebase custom token

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/operations/payments.gql

@@ -63,3 +63,17 @@ mutation CancelSubscription {
     stripeSubscriptionId
   }
 }
+
+mutation CreateAuthHandoffToken {
+  createAuthHandoffToken {
+    token
+    expiresInSeconds
+  }
+}
+
+mutation ExchangeAuthHandoffToken($token: String!) {
+  exchangeAuthHandoffToken(token: $token) {
+    customToken
+    userId
+  }
+}