Consolidate the three API access patterns into a single client module

[priscaenoch]

Problem

The frontend reaches the backend three different ways, which duplicates configuration and error handling:

1. src/api.ts uses a hardcoded base with raw fetch.
2. Several pages call relative /api/... paths directly: src/pages/RpcMetricsDashboard.tsx, src/pages/SetupPage.tsx, src/pages/BatchMultiCall.tsx, src/pages/RateLimitDashboard.tsx, and src/hooks/useTxStatus.ts.
3. src/services/sandbox-api.ts uses its own VITE_API_URL base.

Unifying these is cross-cutting: it touches the new shared client, every ad-hoc caller, and their tests.

What needs to be done

• Create a single API client (src/services/apiClient.ts) that owns the base URL (from the configurable-base work), JSON handling, typed error normalization, and admin auth headers.
• Route all callers through it: src/api.ts, src/services/sandbox-api.ts, src/hooks/useTxStatus.ts, and the four pages listed above.
• Standardize error handling so non-2xx responses throw a typed error with status and message.
• Add or update tests for the client and the migrated callers.

Files

• new src/services/apiClient.ts
• src/api.ts, src/services/sandbox-api.ts, src/hooks/useTxStatus.ts
• src/pages/RpcMetricsDashboard.tsx, src/pages/SetupPage.tsx, src/pages/BatchMultiCall.tsx, src/pages/RateLimitDashboard.tsx
• tests under test/

Acceptance deliverables

• Exactly one module defines the API base and fetch/error behavior.
• No page constructs API URLs or handles fetch errors ad hoc.
• All CI checks pass; the change cannot be merged until CI is green.

Tests to pass

• Unit tests for apiClient: success parsing, non-2xx throwing a typed error, base URL resolution.
• Existing page tests continue to pass after the migration.