fix: restrict NIP-42 AUTH to trusted relays only

[barrydeen]

Summary

• Three-tier AUTH policy: auto-sign for user's own relays (pinned + DM), prompt user for recipient DM delivery relays, silently discard AUTH challenges from all other relays (outbox, scored, hint, random ephemeral)
• Per-relay auth toggle in relay settings (on by default) so users can opt out of AUTH on specific relays while still reading/writing to them
• New AuthApprovalDialog prompts when a DM delivery relay requests authentication, with session-scoped approval memory

Test plan

• Verify AUTH auto-signs for user's own pinned and DM relays
• Verify AUTH challenges from scored/outbox/random ephemeral relays are silently discarded (check logcat for "AUTH challenge discarded" messages)
• Send a DM to a recipient whose DM relay requires AUTH — verify approval dialog appears
• After approving, verify no re-prompt on reconnect within same session
• Toggle auth off for a relay in settings — verify AUTH challenges are rejected for that relay
• Verify existing relay configs deserialize correctly (new auth field defaults to true)