feat: add public profile share card route

[ugoocreates-pixel]

Closes #424

Resolves the issue to add a public profile share card.

Changes

• Added a new PublicProfileCard page component to display mock profile data.
• The route is read-only and renders without authentication.
• Includes a share button that copies the profile URL to the clipboard.
• Handles 404 gracefully when the user handle/id is explicitly "missing" or "not-found".
• Registered the routes /u/:handle and /profile/:id in App.tsx.

[vercel]

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