feat: add dynamic OG meta tags for public profile pages

[bindhurapana-2828]

Summary

Fixes the missing OpenGraph meta tags on public profile pages /u/[username].

Closes #1800

Changes Made

• Created src/app/api/og/user/route.tsx — dynamic OG image using @vercel/og
• Updated src/app/u/[username]/page.tsx — added og:title, og:description, og:image meta tags
• Installed @vercel/og package

Type of Change

• New feature

How to Test

Visit any profile page URL and check link preview on LinkedIn/Twitter/Discord.

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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