fix(mobile): hide archived identities from mentions

[svndco]

What changed

• load the relay identity from its NIP-11 document and query the latest relay-signed NIP-IA kind:13535 archive snapshot
• verify the snapshot signature, signer, and protected-event tag before trusting archived pubkeys
• hide archived identities from channel-member, agent-directory, and user-search mention candidates while preserving the current user
• keep mention suggestions hidden while archive state is loading or refreshing
• add provider, candidate, and composer regression coverage

Why

Buzz Mobile built mention candidates without consulting the relay archive snapshot, so retired agent identities remained visible after they had been safely archived.

Verification

• flutter test: 1,077 passed, 1 skipped
• flutter analyze --no-pub: no issues
• dart format --output=none --set-exit-if-changed lib test: clean
• node scripts/check-file-sizes.mjs: passed
• independent pre-commit review: passed with no blocking correctness or security issues

Originating Buzz channel: adf37e23-abde-4503-87d4-322593a0759e