Fix: Update hero greeting & CTA based on auth state (#765)

[Gourangi4]

Issue Fixed: #765

What changed

• Replaced hard-coded “Welcome Back” with neutral “Welcome to NotesVault” for logged-out users.
• Added JS check using localStorage keys nv_user and nv_user_name.
• CTA logic:
  • Logged out → “Sign Up” (href: signup.html)
  • Logged in → “Upload Notes” (href: upload.html), greeting “Welcome back, ” when available.

Why
Prevents confusing message for first-time/logged-out users; improves onboarding clarity.

How I tested

1. Incognito / cleared storage → shows “Welcome to NotesVault” + “Sign Up”.
2. Console:

   localStorage.setItem('nv_user','1');
   localStorage.setItem('nv_user_name','Gourangi');
   location.reload();

→ shows “Welcome back, Gourangi” + “Upload Notes”.

localStorage.removeItem('nv_user');
localStorage.removeItem('nv_user_name');
location.reload();

→ returns to logged-out state.

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[Gourangi4]

hey @adityai0 , kindly check the pr I have submitted.