docs: Add Angular-to-React frontend migration plan to README

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Summary

Adds a comprehensive Angular → React frontend migration plan section to the README. This repo contains backend-only .NET 10 microservices with no frontend code; the new section documents how to build a React frontend that consumes these APIs through the YARP API Gateway.

Key additions:

• Target tech stack comparison (Angular → React equivalents for framework, routing, state management, testing, etc.)
• Proposed React project structure under src/frontend/
• 6-phase migration roadmap with scope and time estimates (~3–4 weeks total)
• Angular → React concept mapping table (Components, Services, RxJS, Guards, Pipes, etc.)
• Technical notes on currency handling (cents), correlation ID propagation, notification HTML preview, and API gateway integration

Review & Testing Checklist for Human

• Verify the migration phases and time estimates align with your team's capacity and priorities
• Confirm the proposed tech stack choices (React 18 + Vite + MUI + TanStack Query) match your preferences
• Decide on frontend placement: src/frontend/ in this repo vs. a separate repository

Notes

• No code changes — this is a documentation-only update
• The Related Repositories table entry for the monolith was updated to clarify it contains the original Angular frontend

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/c67eaa4163b642f2b33b4c31f49d21a5

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✅ Devin Review: No Issues Found

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