Add phone number's country code drop down can be occluded

**Describe the bug**
As titled.

**Authgear Version**
- self host git-325a7d7e089e

**To Reproduce**
Steps to reproduce the behavior:
1. Enable phone as identity on top of email
2. Create a user with email
3. Go to settings -> identity (more) -> add phone
4. Make the window as narrow as it could be without making the input panel full width
5. Click country code
6. Observe the drop down is occluded (need to scroll to see)

**Expected behavior**
At (6) the dropdown isn't occluded

**Screenshots**

**Client Env (if applicable, please complete the following information):**
- Device: N/A
- OS: Windows
- Browser: Chromium edge
- Browser Version: 07.0.1418.35 (Official build) (64-bit)

**Additional context**
Add any other context about the problem here.

Reproduce the issue through Settings → Identity (more) → Add phone using a narrow window, then locate the country-code dropdown used by that flow. Confirm the fix by checking that the full dropdown remains visible without scrolling at the reported narrow width in Chromium Edge.

Written by the indexing model from the issue text.