restore: local relay search opt-in for logged-in users

[dergigi]

Restore the logged-in local relay opt-in from v0.3.4.

Scope:

• relay panel checkbox for local/private relay search
• only opt in the logged-in user's own relays
• keep the browser safety constraints that motivated the original change

Keep this scoped to the Gigi-built implementation that existed before the reset.