Unify account usage freshness

[yvette-carlisle]

Summary

• make the Web dashboard account pool read only from /api/accounts?refresh=1
• remove runActivity top-level account snapshots so stale lane captures cannot override fresh account usage
• align macOS account refresh to 15s forced refresh and update current-lane tests

Verification

• cargo test -p decodex operator_dashboard --all-features -- --test-threads=1
• cargo test -p decodex operator_state_endpoint --all-features -- --test-threads=1
• swift test --package-path apps/decodex-app
• rustup run nightly cargo fmt --all -- --check
• git diff --check
• scripts/macos/test_decodex_app_stage.sh
• local dashboard smoke on 127.0.0.1:8193