feat (nextjs) Phase 3 - API routes, Server Actions, TypeScript migration

[Amank1412]

Summary

Completes the React → Next.js 15 + TypeScript migration for the frontend (Issue #87). This PR adds API route proxying, Server Actions, and finishes all remaining TypeScript conversions.

With this PR, the full migration is complete all 5 phases done.

What's in this PR

API Routes

• app/api/health/route.ts — Backend health check proxy
• app/api/search/route.ts — Search proxy to Python backend
• app/api/store/route.ts — Metadata store proxy
• app/api/anonymize/route.ts — File anonymization proxy (routes to JS or Python based on file type)
• app/api/ipfs/upload/route.ts — IPFS upload proxy to JS backend
• app/api/preview/[type]/route.ts — Dynamic preview proxy for image/spreadsheet/pdf/dicom
• app/api/proxy/route.ts — General backend proxy

Server Actions

• app/actions.ts — searchDocuments(), storeMetadata(), checkHealth()

TypeScript Conversions

• app/layout.js → app/layout.tsx with Metadata type
• app/page.js → app/page.tsx with typed state (ViewType, typed wallet handlers)
• types/ethereum.d.ts — window.ethereum TypeScript declarations

Build & Config

• tsconfig.json — target updated to ES2018
• tailwind.config.js — content paths now include .ts/.tsx extensions
• next.config.js — image optimization (avif/webp), compression enabled, vendor chunk splitting

Documentation

• README.md — complete rewrite with architecture diagram, full project structure, tech stack table
• MIGRATION.md — detailed file mapping (React → Next.js), key changes, build verification

Related

Closes #87
Follows #90 (Phase 1) and #94 (Phase 2)

[Amank1412]

Hi @pradeeban Sir

All phases for this migration are now complete
This PR resolves Issue #87 Please review when convenient.

Thanks!

[pradeeban]

@Amank1412 there are a lot of merge conflicts. Can you pls resolve them?

[Amank1412]

hi @pradeeban Sir Conflicts resolved. Ready for review.