fix: Auto-fill chat username from Supabase session instead of manual entry

[Ananya-CM]

Summary

Fixed the Chat page which required users to
manually type their name before chatting, even
though they were already authenticated via Supabase.

Root Cause

In frontend/app/chat/page.tsx:

const [username, setUsername] = useState("");

Username was initialized as empty string with
no connection to the authenticated user session.

Fix

Added a useEffect that fetches the real
username from Supabase session on mount:

useEffect(() => {
  const getUsername = async () => {
    if (!supabase) return;
    const { data: { session } } = 
      await supabase.auth.getSession();
    if (session?.user) {
      const name =
        session.user.user_metadata?.full_name ||
        session.user.email?.split("@")[0] ||
        "User";
      setUsername(name);
    }
  };
  getUsername();
}, []);

Priority order for username:

1. user_metadata.full_name (real name)
2. Email prefix before @ (fallback)
3. "User" (final fallback)

Before

• Username input was empty on page load
• User had to manually type their name
• Messages could appear under wrong identity

After

• ✅ Username auto-filled from Supabase session
• ✅ User can start chatting immediately
• ✅ Username input still editable if needed

Screenshots

Files Changed

• frontend/app/chat/page.tsx — only file modified

Issue

Closes #255

nsoc26

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