Recognize ACP registry agent icons

[Finesssee]

Summary

• Prefer secure catalog-provided icons for ACP agents when marketplace or registry metadata includes one
• Keep legacy ACP marketplace IDs mapped to existing provider icons when no catalog icon is available
• Render ACP agent icons in the agent selector and Extensions settings list
• Add focused coverage for registry icon URLs and legacy/current marketplace ID fallbacks

Test Plan

• bunx vp test run src/features/ai/tests/provider-icons.test.ts
• bun typecheck
• git diff --check

Notes

• This is frontend-only and does not change catalog loading, installation, or runtime behavior.

[Finesssee]

Superseded by #682. The ACP runtime/catalog branch now includes the newer provider icon/logo handling and records this branch as superseded, so this PR no longer needs to land separately.

[Finesssee]

Closing as superseded by #682.