docs: add frontend API validation guide

[SHAURYAKSHARMA24]

Summary

Closes #132.

Adds frontend/API validation documentation for OpenShield covering the React/Vite frontend, Flask API endpoints, JWT behaviour, CORS/error handling, live mode setup, and frontend-to-backend data flow.

Changes

• Added docs/validation/FRONTEND_API_TESTING.md
• Added docs/validation/TEST_RESULTS.md
• Documented frontend page inventory
• Documented page-to-API endpoint mapping
• Documented live mode setup using VITE_API_URL and JWT token handling
• Documented expected API response shapes
• Added auth/JWT validation checklist
• Added CORS, loading, error, and empty-state validation checklist
• Added known frontend/API gaps and recommended follow-up issues
• Added structured frontend/API test results table

Validation Results

• npm install: Pass
• npm run lint: Failed — 65 lint errors, mostly unused React imports
• npm run build: Pass
• npm run dev: Pass — Vite starts on localhost:5173
• pytest: Failed — DATABASE_URL not configured for PostgreSQL-backed tests

Key Findings

• All eight API blueprints are registered in api/app.py
• GET route authentication behaviour appears inconsistent with older frontend/API documentation
• Several frontend pages lack proper error handling and may remain stuck in loading state if API calls fail
• frontend/API_ENDPOINTS.txt appears outdated in areas including port references, AI endpoint paths, demo/live mode description, and endpoint status
• API/database validation requires a configured PostgreSQL DATABASE_URL

Security

No secrets, JWT values, Azure credentials, database credentials, .env files, or local environment files were committed.