test: Add comprehensive unit tests for React hooks and stores

[AugistineCreates]

This pr closes #6

This PR introduces comprehensive unit tests for core React hooks and custom stores to improve test coverage and ensure stability.

Changes Made

• Configured TypeScript Jest transformations and coverage settings in jest.config.js.
• Added a Node-based execution path for Jest tests in package.json.
• Added comprehensive test suites for:
  • useApi
  • useContractMutation
  • useConfirmedMutation
  • useTransactionPreview
  • useRepaymentOperation
  • useWalletStore
• Created standard React Query test utilities (test-utils.ts).

[ogazboiz]

appreciate the effort on test coverage, but this can't land as-is, it regresses the suite from 1 failing test to 12 failed suites / 49 failed tests. the jest config changes are the core problem:

1. jest.config.js:10 replaces testEnvironment: "jest-environment-jsdom" with a hand-rolled transform, so every DOM test loses jsdom and now fails with "document is not defined" (Button.test.tsx, AmountInput.test.tsx, RemittanceForm.test.tsx, ErrorBoundary.test.tsx, useModalFocusTrap.test.tsx, stores.test.ts). keep testEnvironment: "jest-environment-jsdom" and drop the custom transform (next/jest already provides one).
2. jest.config.js:10 transform regex '^.+\.(ts|tsx)$' has doubled backslashes so it never matches .ts/.tsx. remove the transform line entirely.
3. @testing-library/react-hooks is imported by useConfirmedMutation.test.ts:2, useContractMutation.test.ts, useRepaymentOperation.test.ts:2, useTransactionPreview.test.ts but it's not in package.json and is deprecated/react-19-incompatible. rewrite those to use renderHook/act from @testing-library/react (already a dep).
4. useTransactionPreview.test.ts and useContractMutation.test.ts are syntactically corrupted (quotes replaced by backslashes). rewrite the imports/string literals with proper quotes.
5. jest.config.js:17-24 adds a coverageThreshold 75/80 that fails the run regardless (actual ~37%). drop it (or set to current) on a test-adding PR.
6. test-utils.ts uses cacheTime (TanStack v5 renamed it gcTime).

the two files that DO run and assert real behavior (useWalletStore.test.ts, useApi.test.ts) are a good base. get the config + the broken files sorted so the suite runs green and i'll take another pass.

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