feat: upgrade Node.js from 8.15.1 to 20.18.0 LTS (SCRUM-2)

Summary

This PR upgrades the Node.js runtime from the severely outdated version 8.15.1 (EOL since December 2019) to Node.js 20.18.0 LTS across both bank-client and bank-server repositories. This critical infrastructure upgrade enables modern dependency updates, security patches, and improved performance.

Key Changes:

• Updated .nvmrc files to specify Node.js 20.18.0
• Updated package.json engines field to require Node.js >=20.18.0
• Created missing .nvmrc file for bank-server
• Added missing engines field to bank-server package.json
• Regenerated yarn.lock files with Node.js 20.18.0 and Yarn 1.22.22
• Verified both applications start successfully with the new Node.js version

Testing Performed:

• ✅ Bank-client starts on localhost:3000 with NODE_OPTIONS="--openssl-legacy-provider"
• ✅ Bank-server starts in development mode with file watching
• ⚠️ TypeScript compilation shows 393 errors (toolchain compatibility issues)
• ⚠️ Webpack build shows modern JS syntax parsing errors (webpack 4.x + Node 20.x compatibility)

Review & Testing Checklist for Human

Risk Level: 🟡 MEDIUM - Major version upgrade with some compatibility warnings

• Full application functionality test: Verify all features work end-to-end, not just startup
• Production build verification: Ensure npm run build works correctly for client
• Legacy provider requirement: Confirm client requires NODE_OPTIONS="--openssl-legacy-provider" and this is acceptable
• TypeScript/webpack errors: Evaluate if the 393 TypeScript errors and webpack build warnings affect functionality
• Performance validation: Compare application performance before/after upgrade

Recommended Test Plan:

1. Start both applications locally with Node.js 20.18.0
2. Test core banking features: login, account creation, transactions, dashboard
3. Run production builds and verify they complete successfully
4. Consider load testing if performance is critical

Diagram

%%{ init : { "theme" : "default" }}%%
graph TB
    subgraph "akkp-windsurf/bank-client"
        A["client/.nvmrc<br/>lts/dubnium → 20.18.0"]:::major-edit
        B["client/package.json<br/>engines: >=8.15.1 → >=20.18.0"]:::major-edit
        C["client/yarn.lock<br/>Regenerated with Node 20.x"]:::major-edit
    end
    
    subgraph "akkp-windsurf/bank-server"  
        D["server/.nvmrc<br/>Created: 20.18.0"]:::major-edit
        E["server/package.json<br/>Added engines: >=20.18.0"]:::major-edit
        F["server/yarn.lock<br/>Regenerated with Node 20.x"]:::major-edit
    end
    
    G["Node.js Runtime<br/>8.15.1 → 20.18.0 LTS"]:::context
    
    A --> G
    B --> G  
    D --> G
    E --> G
    
    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

Notes

• OpenSSL Legacy Provider: The client application requires NODE_OPTIONS="--openssl-legacy-provider" due to webpack 4.x compatibility issues with Node.js 20.x's updated OpenSSL
• TypeScript Errors: The server shows 393 TypeScript compilation errors related to type definitions, but the application starts and runs successfully
• No CI Updates: No GitHub workflow files were found in either repository, so no CI configuration updates were needed
• Yarn Version: Upgraded to Yarn 1.22.22 (latest stable) for improved package resolution

Session Details:

• Link to Devin run: https://app.devin.ai/sessions/f536c416ddfb4d29a3b384fe02330fc3
• Requested by: Arthur Poon (@akkp-windsurf)
• JIRA Ticket: SCRUM-2