fix(mock-client): unify mock client implementation and add pagination support (#588)

[ravendevhub]

Summary

Resolves #588 by consolidating the mock client implementation in src/lib/mock-client.ts, adding proper pagination slicing to getHistory, isolating instance state, and ensuring main.tsx uses the canonical createMockClient factory.

Changes

1. Pagination Support in getHistory:
   • Updated MOCK_HISTORY generation to 25 items so multi-page pagination can be exercised.
   • Sliced transactions in getHistory using page (1-indexed) and limit.
   • Returned accurate total count (MOCK_HISTORY.length) to enable page calculations in TransactionHistory.
2. State Isolation:
   • Enclosed mutable variables (activeNetwork, connectedAddress) within the createMockClient() closure, eliminating module-level state cross-contamination between test cases.
3. Documentation:
   • Added JSDoc explaining the canonical mock client architecture and strategy.
4. Unit Tests:
   • Added unit tests in src/lib/mock-client.test.ts and src/lib/__tests__/mock-client.test.ts covering multi-page pagination slicing, distinct page datasets, and instance state isolation.

Acceptance Criteria

• There is exactly one source of mock client data (mock-client.ts)
• mock-client.ts is actively imported and used by main.tsx
• getHistory respects page and limit (page 2 returns different results than page 1)
• TransactionHistory Prev/Next pagination buttons navigate through distinct pages of data
• The chosen approach is documented with a comment explaining the mock strategy