Keep Runtime metrics responsive across tabs

[sam2tom]

Summary

• keep Session Metrics mounted so tab switches retain the last chart and prefetch history
• refresh durable history independently every 30 seconds instead of cancelling on each live snapshot
• render unavailable chart values as zero at presentation time while preserving raw history semantics and the durable uptime boundary

Validation

• Web TypeScript check
• Web test suite: 65 files / 647 tests
• Vite production build
• git diff --check
• independent blind review: no blockers
• deployed and health-checked on the authorized test host

Boundaries

Web-only follow-up to merged PR #51. No Core API, database, ClickHouse, or backend changes.

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