Stabilize desktop-edge dev flow and fallback auth handling

[IgnazioDS]

Summary

• stabilize desktop-edge dev flow and fallback auth handling
• includes desktop runtime/config updates and edge auth/rate-limit fallback behavior changes
• extends edge test coverage for localhost guard, frame limits, fallback behavior, and rate-limit scenarios

Local validation run in this workspace

• make check-desktop-rust ✅
• make demo-up ❌ (admin proxy check failed at /api/cloud/v1/admin/stats while direct cloud admin endpoint returned 200)
• make release-check ❌ (same blocker via demo readiness: demo-up)

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.