feat(auth): integrate Supabase Auth and User Profile management #6

Summary

Implement Supabase Authentication integration and Community Impact Profile management for the Next.js web application.

Scope

• Configure @supabase/ssr helpers in lib/supabase/ (client.ts, server.ts, middleware.ts).
• Implement user authentication routes in app/(auth)/:
  • Sign in / Sign up flow (Email & optional GitHub OAuth).
  • Session refresh and route protection via Next.js middleware.
• Profile management:
  • User profile creation trigger on signup.
  • Profile edit page (app/dashboard/profile/page.tsx) supporting display name, bio, GitHub link, and public/private visibility toggle.
  • Public Community Impact Profile view (app/(public)/profiles/[username]/page.tsx).

Acceptance Criteria

• Authentication flows work end-to-end.
• Protected routes redirect unauthenticated users cleanly.
• Public vs private profile visibility respects RLS and user settings.