fix(mobile): Apply native header options only while focused#5

What Changed

Screens on the mobile app now update their native navigation header only while they are the focused screen. A screen that changes its header options while hidden applies them when it regains focus, and every return to a screen reapplies its current header once. Caller-provided listeners such as blur cleanup keep working, and all listeners are removed on unmount.

Why

Hidden screens could call into native navigation during a push or pop. On iOS that produced "Unbalanced calls to begin/end appearance transitions" and left the Home list painted but unresponsive to taps. Limiting header writes to the focused screen removes that re-entry without remounting the navigator.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes