perf(explorer): make homepage data load independently

[moscowchill]

Summary

• Render homepage statistics and tables as their own responses arrive, while keeping the exact transaction total independent.
• Use the existing timestamp index before the unchanged canonical-block filter for network transaction lists. Pagination and limits remain after canonical validation.
• Preserve available data during failed refreshes, show explicit unknown/error states, and cancel stale requests without changing foreground-only polling.

Validation

• Frontend: 583 tests, type checking, lint with zero errors, scoped formatting and production build passed.
• Production-bundle browser checks: deferred exact count, independently visible blocks and transactions, delayed overview, retained data after failed refresh, hidden-tab polling suppression, foreground recovery and mobile layout passed.
• Backend: full Go tests, vet, static build and focused race checks passed.
• Isolated MongoDB regression tests cover missing, incomplete and conflicting blocks, pagination/defaults/limits, timestamp ties, unchanged decoded responses and exact counts.

Deployment and safety

The API response shape, exact count semantics and canonical visibility checks are unchanged. No database index or schema migration is required. Deployment will retain the previous frontend and API artifacts for rollback, followed by public fresh-load and endpoint timing checks.