Expand unit test coverage for untested pages

[priscaenoch]

Problem

There are only 8 test files under test/ for roughly 114 source modules in src/. Several core pages have no tests at all: ContractPage, EventPage, GraphPage, Sandbox, BatchMultiCall, RpcMetricsDashboard, RateLimitDashboard, and SubInvocationPage. Meaningfully closing this gap is a broad, multi-file effort spanning a new test file per page, shared mocking of the API client, and coverage wiring.

What needs to be done

• Add Testing Library tests for each untested page listed above, mocking the API client: cover initial render, loading state, successful data render, and error state.
• Add tests for critical utilities in src/utils (formatting, XDR helpers) that currently lack coverage.
• Ensure the shared API client is exported/mockable to support these tests.
• Wire coverage thresholds so the added coverage is enforced (coordinate with the coverage-provider issue).

Files

• new test files under test/ (one per page listed above)
• new tests for src/utils/*
• src/services/apiClient.ts (export for mocking, if needed)
• vitest.config.ts (coverage thresholds)

Acceptance deliverables

• Each listed page has at least render, loading, and error tests.
• Test count meaningfully increases and npm test passes.
• All CI checks pass; the change cannot be merged until CI is green.

Tests to pass

• The new page and utility tests pass in CI.
• Coverage does not regress below the configured threshold.