Fix search navigation button initial states

[Copilot]

Swift 6 language mode no longer triggers didSet observers for property assignments inside defer blocks during initialization. The original defer { isEnabled = false } pattern in RoundedButtonGroup.init relied on this undocumented Swift 5 behavior to propagate disabled state to child buttons.

// Swift 5: didSet fires → buttons updated ✓
// Swift 6: didSet suppressed → buttons stay enabled ✗
defer { isEnabled = false }

• Extract button state propagation into updateAppearance(), called from both didSet and explicitly at end of init
• Remove defer trick, set default value to false directly
• Call updateAppearance() explicitly after init setup completes

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.