feat(dashboard): add loading skeletons for widgets

[byteAbhi]

Summary

Adds loading skeleton states for dashboard widgets to improve perceived performance and reduce layout shifts while dashboard data is loading.

Closes #1774

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• Added loading skeleton UI for ContributionGraph.tsx
• Added skeleton states for PRMetrics.tsx, TopRepos.tsx, StreakTracker.tsx, and GoalTracker.tsx
• Improved dashboard loading UX by reducing layout shifts and visual flicker

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How to Test

1. Start the application locally using pnpm dev
2. Open the dashboard page and throttle network speed (optional: Slow 3G in browser dev tools)
3. Refresh the page and observe skeleton loaders before actual data loads

Expected result:

Dashboard widgets should display skeleton placeholders during loading and smoothly transition to actual content once data is fetched.

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Screenshots / Recordings

| Before

| After

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed
• Tested on mobile / responsive layout

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Additional Context

This PR was recreated from a clean branch based on the latest upstream main because the previous branch accumulated merge conflicts during rebase. The functionality remains unchanged.

[github-actions]

GSSoC Label Checklist 🏷️

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

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• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:exceptional — ×1.5 multiplier

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[byteAbhi]

Recreated this PR from a clean branch based on latest upstream/main because the previous branch accumulated heavy merge conflicts during rebase. This PR contains the same feature changes with clean commit history.

[Priyanshu-byte-coder]

Hi — new UI components/features are currently on hold while we focus on code quality (see discussion #2651). Loading skeletons are a nice addition, but please hold this until the quality sprint wraps up. PRs for tests, refactoring, bug fixes, and docs are welcome in the meantime.

[Priyanshu-byte-coder]

New feature/UI PRs are on hold during the code-quality sprint — see discussion #2651. We'll revisit this after the sprint; meanwhile tests, refactors, and bug-fix PRs are very welcome.

[Priyanshu-byte-coder]

Unified widget skeletons just landed via #2639 (shared WidgetSkeleton + SkeletonBlock), which covers this. Closing — thanks for the work here, and please build on the new shared component for any remaining widgets.