Contacts: signed introductions and trusted visor meetings

[lannbot]

Summary

• Add signed Protobuf introductions with issuer self-claims, multiple parties, timestamps, exact-byte verification, and bounded decoding.
• Add a private encrypted contacts document with a user-level signing identity, separate meeting provenance, local petnames and preferred values, selective import, and explicit keyless-contact merging. Verified introduction payloads/signatures never enter persisted contact history.
• Add trusted visor contact management, claim-selection previews, portable contact files/links, and interactive QR/link meetings with comparison codes and mutual confirmation. Meetings use a separate protocol and never enroll peers into the user's device group.
• Replace horizontal drawer transitions with a responsive left sidebar contained inside the drawer; strip actions remain direct navigation shortcuts.
• Add a durable checkpoint completion barrier for contact mutations, including coalesced-write failure propagation.

Validation

• just wit
• cargo fmt --check
• cargo test --workspace, plus targeted contact/durable-checkpoint and visor tests after final changes
• cargo clippy --workspace --all-targets -- -D warnings
• deno task check and deno task test (36 tests)
• just build-wasm and just site-fixtures
• Real Chromium: all 36 scenarios pass across the full run and targeted reruns after fixes. Includes portable selective import/tamper rejection, independent-user meetings, responsive sidebar geometry/focus/touch targets, and existing app/pairing/Drive flows.
• Independent reviews of model, meeting protocol, kernel integration, and UI; blocking findings resolved.

Automerge will be armed after opening.