fix(desktop): show accessible relay agents in Agents and Huddles

[dpwai-admins]

Summary

• replace the local starter/demo-only Agents view with the relay agents the current identity can actually instruct
• reuse the existing verified-owner, allowlist, and shared-channel admission policy
• list relay agents in the Huddle chooser and keep already-enrolled agents visible as disabled rows
• load relay profile avatars and preserve private access boundaries
• display the viewer's full npub prominently on the Agents page with separate npub and hex copy actions
• show an actionable warning when an enrolled agent cannot receive voice because the microphone is unavailable/muted or transcription is disabled

Why

A relay agent can be registered, online, and able to reply in a DM while remaining absent from both the Agents page and the Huddle picker. Those surfaces were reading only the local managed-agent directory. Already-added agents were also silently filtered out of the Huddle picker, which made a valid enrollment look missing.

Huddles default to push-to-talk with the microphone muted. Agents consume the STT transcript rather than raw Opus audio, so this previously looked like an enrolled agent was ignoring the user while no transcript was ever published.

The Agents page is also the natural place to make the current user's public identity unambiguous. It now shows only the public npub/hex; no private-key material is read or rendered.

Verification

• TypeScript: pnpm --dir desktop typecheck
• Biome: all 9 touched files clean
• focused latest-change tests: 9 passed
• full desktop suite before the isolated voice/pubkey UI additions: 5,310 passed, 0 failed
• production frontend after all three commits: pnpm --dir desktop build

Draft because distribution still requires the project's official signed macOS release pipeline.