Hide run strip controls unless overflowing

[yvette-carlisle]

Summary

• only render active-lane strip edge controls when the lane chips overflow the full no-controls width
• keep both edge controls visible while overflowing, with disabled styling at the start/end instead of disappearing
• keep fades and continuous scrolling disabled when controls are hidden

Verification

• swift build --package-path apps/decodex-app
• swift test --package-path apps/decodex-app
• git diff --check