fix(mobile): hide archived identities from mentions

[joshpalerlin]

What

• load the active relay's NIP-11 self identity
• fetch and verify its NIP-IA kind:13535 archive snapshot
• remove archived identities from mobile channel members, relay-agent suggestions, and global mention search results
• keep the current-user exemption used by desktop to avoid self-shadowing
• refresh the picker when a newer relay-signed snapshot arrives

Why

Buzz Desktop already excludes archived identities from mention autocomplete. Mobile did not load the relay archive snapshot, so retired owned agents could remain visible beside their active replacement.

Safety

• archive state is scoped to the active relay/community
• snapshots must match the relay identity advertised by NIP-11
• event id and Schnorr signature are verified before use
• exactly one NIP-70 marker is required
• malformed or unavailable state fails open without hiding identities
• NIP-01 replacement ordering handles newer and same-timestamp snapshots

Proof

• dart format --output=none --set-exit-if-changed .
• flutter analyze — no issues
• focused mention/archive tests — 29 passed
• full flutter test — 1,584 passed

Handoff

• Source: official NIP-IA client behavior plus the existing desktop archive-aware mention implementation
• Current gate: build and Docker verification complete; ready for maintainer review
• Next step: maintainers review the mobile provider shape and CI confirms the branch