## Summary
- fix Step 5 field detection to prefer visible `input[name="age"]`
- stop treating hidden `input[name="birthday"]` as a birthday-mode signal
- preserve birthday-mode handling for visible birthday controls
## Problem
On some pages, a visible age input and a hidden birthday input coexist.
The previous logic treated the hidden `input[name="birthday"]` as evidence of birthday mode, which caused Step 5 to
fill birthday instead of age.
## Verification
- reproduced on a page where `input[name="age"]` is visible
- confirmed `input[name="birthday"]` exists but is hidden
- verified Step 5 now enters the age branch and fills the age input correctly
