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 BASE = "/api" with raw fetch.
2. Several pages call relative paths directly, e.g. fetch("/api/rpc-metrics") (src/pages/RpcMetricsDashboard.tsx), fetch("/api/setup/doctor") (src/pages/SetupPage.tsx), fetch("/api/batch/simulate") (src/pages/BatchMultiCall.tsx), fetch("/api/admin/analytics/...") (src/pages/RateLimitDashboard.tsx), useTxStatus.ts.
3. src/services/sandbox-api.ts uses import.meta.env.VITE_API_URL with its own base.

What needs to be done

• Create a single API client (e.g. src/services/apiClient.ts) that owns the base URL (from the configurable-base issue), JSON handling, error normalization, and (optionally) auth headers for admin routes.
• Route all existing calls through it: src/api.ts, the inline fetch calls in the pages listed above, useTxStatus.ts, and sandbox-api.ts.
• Standardize error handling: non-2xx responses throw a typed error with status and message.

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

Acceptance deliverables

• Exactly one module defines the API base and fetch/error behavior.
• No page constructs API URLs or handles fetch errors ad hoc.

Tests to pass

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