feat(dashboard): add Twitter/LinkedIn share buttons with live stats (#2265)

[yachikadev]

Summary

Adds Phase 2 of the shareable stats feature requested in #2265 — Twitter/X and LinkedIn share buttons alongside the existing "Share Profile" (copy link) button.

Changes

• DashboardHeader.tsx: fetches user's streak, PR merge rate, and weekly goal progress, and passes them to ShareProfileButton as a stats prop.
• ShareProfileButton.tsx: adds a Twitter/X button that opens a pre-filled tweet with the user's stats and profile link, and a LinkedIn button that opens the LinkedIn share dialog with the profile URL. Copy-link (Phase 1) behavior is unchanged.

Testing

• Verified copy link still works and shows "Copied" confirmation.
• Verified Twitter/X button opens intent URL with correct pre-filled text including live stats.
• Verified LinkedIn button opens share dialog with correct profile URL.
• Checked both desktop and mobile dropdown render paths.

Notes

• Stat field names (current_streak, merge_rate, progress_percent) are assumed based on existing API routes — happy to adjust if actual response shapes differ.
• Phase 3 (downloadable PNG stats card) is a stretch goal from the issue and is not included in this PR; can be tackled separately if desired.

Closes #2265

[github-actions]

GSSoC Label Checklist 🏷️

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