Remove blurred edge above Focus URL bar

[ctbeiser]

📜 Tickets

N/A - Web Viewer customization.

💡 Description

Uses the clear UIGlassEffect style for the Focus URL bar on iOS 26 and newer, removing the regular glass style's blurred edge above the URL. Updates CUSTOMIZATIONS.md with the rendering choice.

🎥 Demos

Verified before and after on an iPhone 17 Pro simulator running iOS 26.5.

📝 Checklist

• I filled in the ticket numbers and a description of my work (ticket not applicable)
• I updated the PR name to follow our PR naming guidelines
• The Focus app builds; no new logic requires unit tests
• UI semantics and accessibility behavior are unchanged
• No telemetry was added or modified
• No strings were added or modified
• I updated the customization documentation