Record paired device last connection time

[enaboapps]

Closes #643

What changed

• turn every authenticated connection.ping into a connection event with the validated device ID and desktop timestamp
• save lastSeenAt and an optional Remote name in one transaction before publishing either value in memory
• reject out-of-order connection events so stale Windows tasks cannot replace newer metadata
• handle the event through the Windows and macOS runtimes
• show "Last connected" in the Devices view

Raw Bluetooth subscriptions and failed authentication do not update device metadata. Existing ping acknowledgements and the name_update_failed response remain compatible.

Validation

• Node.js 24.13.0
• Rust 1.97.1
• npm run lint
• npm test (74 tests)
• npm run build
• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path src-tauri/Cargo.toml (211 unit tests, 2 config tests)

[enaboapps]

Independent exact-head review completed for 3852b50a74dc9c36791d87db3577b9752dbcb73f.

The reviewer confirmed the earlier out-of-order Windows persistence finding is fixed and reported no actionable findings on the updated head. Frontend, macOS app/DMG, and Windows NSIS CI all pass.