fix: #536 unify mock client adapter and support pagination in getHistory

[jotel-dev]

Description

Resolves #536

Problem

src/lib/mock-client.ts provided createMockClient(), but src/main.tsx previously created an inline adapter with hardcoded responses and ignored pagination parameters in getHistory, causing mock divergence and broken pagination navigation in TransactionHistory.

Solution

1. Configured src/main.tsx to use createMockClient() from src/lib/mock-client.ts as the single authoritative mock source and added code comments documenting the client strategy.
2. Updated getHistory in src/lib/mock-client.ts to respect page and limit parameters for pagination slicing, while returning the full count as total.
3. Expanded MOCK_HISTORY to 25 items so multi-page pagination controls (Prev/Next) function in the demo interface.
4. Added unit tests in src/lib/mock-client.test.ts to verify multi-page slicing, non-overlapping slices, and total counts.

Testing

• npx vitest run src/lib/mock-client.test.ts src/lib/__tests__/mock-client.test.ts (All 20 tests pass)
• npx vitest run src/App.test.tsx (All tests pass)
• npm run typecheck (Passes with 0 errors)
• npm run build & npm run build:app (Both build successfully)

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