feat(auth): dedicated profile onboarding screen for new users

[trtajim]

🚀 Feature Summary

1. Dedicated Onboarding View (/onboarding):

   • Modern, distraction-free card with minimal branding and dark mode support.
   • Collects 3 essentials: Full Name, Username (@handle), and College / School Name.
   • Includes quick-pick chips for popular institutions (Notre Dame College, Dhaka College, etc.).

2. Real-time Handle Suggestions & Live Availability:

   • Automatically generates a clean, unique base username suggestion from the user's name/email.
   • Provides instant live availability checking via /api/check-username with live feedback (Available ✅ / Taken ❌).
   • Live public profile URL preview (hscstack.com/u/handle).

3. Seamless Intended Redirect Handling:

   • Seamlessly preserves Laravel's intended destination (session('url.intended')). When users click login from /chat or any page, completing onboarding sends them directly to where they wanted to go.