feat: add public profile comparison feature

[ramlal-01]

Summary

closes #1068

Adds a public profile comparison system to DevTrack.

Users can now compare themselves with another public DevTrack profile directly from /u/[username].

This introduces:

• shareable comparison URLs
• side-by-side public stat comparison
• winner/tie indicators
• privacy-safe public profile comparisons

---

Features Added

Public Profile Compare Button

On /u/[username]:

• Logged-out visitors see Log in to compare
• Logged-in visitors viewing another profile see Compare with me
• Logged-in visitors viewing their own profile do not see the compare button

Clicking the compare button navigates to:

/compare/currentUser-vs-profileUser

Example:

/compare/priyanshu-vs-ramlal

---

Shareable Compare Route

Added:

src/app/compare/[users]/page.tsx

Supports routes like:

/compare/alice-vs-bob

The compare page:

• works without authentication
• fetches both users using public profile data
• shows side-by-side comparison
• highlights winners/ties per metric
• respects privacy/public profile visibility

---

Architecture Changes

Reused Existing Public Profile Infrastructure

Instead of creating a separate comparison backend, this PR reuses and extends the existing public-profile architecture.

Shared Public Profile Logic

Moved reusable public profile fetch logic into shared helpers to reduce duplication and keep compare pages public-first and privacy-safe.

Public Compare Data

Added support for:

• public PR counts
• public top languages
• reusable compare-ready public stats

---

Privacy Handling

If either profile:

• is private
• unavailable
• missing

the compare page shows an unavailable state instead of exposing hidden data.

---

Files Added / Updated

Added

src/app/compare/[users]/page.tsx

Updated

src/app/u/[username]/page.tsx
src/lib/public-profile-data.ts
src/app/api/public/[username]/route.ts
src/lib/supabase.ts

---

Validation

Verified locally:

npm run lint
npm run type-check
npm run build

All passing successfully.

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[github-actions]

GSSoC Label Checklist 🏷️

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

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[ramlal-01]

Rebased with latest main and re-ran CI.

Build, lint, and typecheck are passing successfully.
The remaining Playwright failure appears to be an upstream test environment issue.

Feature implementation is complete and ready for review. 🚀

[Priyanshu-byte-coder]

CI is failing on this PR. Please fix the failing checks before this can be merged.

[ramlal-01]

Rebase with the main branch.
All checks are now passing successfully.
@Priyanshu-byte-coder

[ramlal-01]

@Priyanshu-byte-coder
please review and merge