feat: Implement comprehensive backend unit testing

[devin-ai-integration]

feat: Implement comprehensive frontend unit testing

Summary

This PR implements comprehensive unit testing for the React/Redux frontend banking application, adding 62 passing tests with 100% coverage for utility functions and extensive test coverage for authentication components, navigation, dashboard widgets, and Redux state management.

Key additions:

• Helper Functions: Complete test suite for 9 utility functions (formatting, validation, string manipulation)
• Authentication Components: LoginForm and RegisterForm with form validation and Redux integration testing
• Dashboard Widgets: AvailableFunds, Bills, and other dashboard component tests
• Redux Layer: Action creators, reducers, saga functions, and selectors testing
• Navigation: App navigation components with route integration tests

Infrastructure improvements:

• Fixed extensive linting issues that were preventing test execution
• Added missing test dependencies (redux-mock-store, redux-thunk)
• Corrected multiple test expectations to match actual function behavior
• Implemented proper mocking for window.matchMedia and styled-components

Review & Testing Checklist for Human

• Critical: Manually test authentication flows (login/register) to verify test coverage aligns with actual functionality
• Critical: Review helper function implementations vs test expectations - multiple corrections were needed during development
• Important: Spot-check Redux saga tests to ensure async operations are properly mocked and tested
• Important: Verify dashboard widget rendering matches test assertions by running the application
• Recommended: Run yarn test --coverage to confirm all tests pass and review coverage report

Suggested Test Plan:

1. Start the application locally and verify login/register forms work as expected
2. Navigate through dashboard to ensure widgets render correctly
3. Test form validation scenarios manually to compare with test expectations
4. Check that helper functions behave as described in their tests

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    subgraph "Test Files (New)" 
        HT["app/helpers/__tests__/<br/>formatting.test.js<br/>validation.test.js<br/>index.test.js"]:::major-edit
        LT["app/components/LoginForm/<br/>tests/index.test.js"]:::major-edit
        RT["app/components/RegisterForm/<br/>tests/index.test.js"]:::major-edit
        DT["app/containers/DashboardPage/<br/>tests/widgets.test.js"]:::major-edit
        ReduxT["app/containers/*/tests/<br/>redux.test.js<br/>saga.test.js"]:::major-edit
    end
    
    subgraph "Source Code"
        Helpers["app/helpers/index.js<br/>(formatBill, validation, etc.)"]:::context
        LoginForm["app/components/LoginForm/<br/>index.js"]:::context
        Dashboard["app/containers/DashboardPage/<br/>index.js"]:::context
        Redux["app/containers/*/reducer.js<br/>saga.js, actions.js"]:::context
    end
    
    subgraph "Config & Dependencies"
        Package["package.json<br/>(added test deps)"]:::minor-edit
        Lint["Various files<br/>(linting fixes)"]:::minor-edit
    end
    
    HT -->|"tests"| Helpers
    LT -->|"tests"| LoginForm  
    DT -->|"tests"| Dashboard
    ReduxT -->|"tests"| Redux
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB  
    classDef context fill:#FFFFFF

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Notes

⚠️ Important: Multiple test expectations required correction during implementation, indicating potential misalignment between test assumptions and actual function behavior. Pay special attention to helper function tests.

Testing Infrastructure: This PR resolves long-standing linting issues that were preventing the test suite from running. The extensive linting fixes should be reviewed to ensure no unintended logic changes were introduced.

Session Details:

• Link to Devin run: https://app.devin.ai/sessions/1c676e6d043148978c8ea8ee52973318
• Requested by: @akkp-windsurf

[devin-ai-integration]

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