Polish ACP agent selector scrolling

[Finesssee]

Summary

• Make the ACP agent selector taller and scrollable enough for the expanded provider catalog
• Render provider icon URLs with colored presentation and keep the existing provider icon fallback
• Keep keyboard selection anchored in view while moving through the full provider list

Validation

• bun typecheck
• bunx vp check src/features/ai/components/selectors/agent-selector.tsx
• git diff --check
• Verified in Athas Dev with Computer Use: expanded provider list is visible, colored icons render, and tail providers like VT Code, Qwen Code, and siGit Code are reachable through search

[Finesssee]

Superseded by #682. I folded the selector scroll polish into the ACP runtime/catalog branch so the selector file no longer needs a separate PR that conflicts with the runtime work.

[Finesssee]

Closing as superseded by #682, which now includes this selector polish on top of the ACP runtime/catalog changes.