[FEAT] Add shareable public profile card — generate a read-only stats URL for social sharing

[Nareshkumawat-star]

Is your feature request related to a problem?
Developers want to showcase their coding consistency. DevTrack has the data but no way to share it — stats live behind auth and can't be linked externally.

Describe the solution you'd like

New route: src/app/u/[username]/page.tsx — public, no auth required

• Shows: GitHub avatar, username, current streak, longest streak, total contributions (last 30d), PR merge rate, top 3 repos
• Minimal, clean card layout (no editing, no goals — display only)
• User must opt-in via a "Make profile public" toggle in settings

OG image: src/app/u/[username]/opengraph-image.tsx

• Use Next.js ImageResponse API to generate a dynamic OG card
• Shows streak flame icon, contribution count, current streak
• Shareable directly on Twitter/LinkedIn/GitHub profile README

"Share my stats" button on the main dashboard → copies the public URL to clipboard

Privacy: opt-in only, default private. User can toggle off at any time.

Supabase schema: add is_public BOOLEAN DEFAULT false to the users table

[Nareshkumawat-star]

/assign

[github-actions]

Assigned to @Nareshkumawat-star! You have 7 days to open a PR. No PR after 7 days = auto-unassigned so others can pick it up. Good luck!