feat: improve monitoring page usability

[zhoutaoo]

Summary

• enrich online-user rows with activity state, online duration, recent activity, and client summary
• keep gateway monitoring sections independently available when one data source fails
• explain unavailable route metrics and runtime snapshots instead of showing ambiguous empty data
• move service instance monitoring into a dedicated details dialog
• refresh the repository design contract

Validation

• pnpm type-check
• pnpm exec vitest run tests/unit/views/online-user-presentation.test.ts tests/unit/views/gateway-monitoring.test.ts tests/unit/views/application-metrics.test.ts
• lint-staged hooks passed