feat(web): add missing middleware.ts — auth guard was never executing

Fix Next.js 16: restore @tailwindcss/postcss, middleware→proxy.ts, serverExternalPackages top-level, add .gitignore

v3: All missing features — auth callback, settings, PDF export, email (Resend), JWT auth, /api/history, SaulLM integration, extension icons, Supabase 0.10 breaking changes, Stripe v22

🛡️ ClauseGuard v1.0 — Full codebase: Extension + Website + API + ML