feat(languages): upgrade Language Breakdown widget to Recharts donut chart (#938)

[mallya-m]

Summary

Closes #938 — upgrades the Language Breakdown widget from a stacked bar to a Recharts donut chart with interactive tooltips and a clean legend.

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Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

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Changes Made

• src/components/LanguageBreakdown.tsx — single file change
  • Replaced the stacked bar chart with a Recharts PieChart (donut style) using innerRadius/outerRadius for the ring shape
  • Added a centred label inside the donut showing the number of languages
  • Added a LanguageTooltip component that shows language name and percentage on hover
  • Expanded the GitHub language color map with 10 additional languages (Swift, Kotlin, C++, C, C#, PHP, Dart, Scala, Vue, Other)
  • Kept the Other bucket logic for languages beyond the top 6 (percentage remainder grouped automatically)
  • Improved empty state: bordered dashed container instead of plain text
  • Improved loading skeleton: circular placeholder matching donut shape
  • Responsive layout: donut + legend stack vertically on mobile, sit side-by-side on sm+
  • No new dependencies — recharts was already installed in the project

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How to Test

1. Log in and navigate to the dashboard
2. Scroll to the Goals & Insights section → Language Breakdown widget
3. Confirm a donut chart renders with coloured slices per language
4. Hover over a slice — tooltip shows language name and percentage
5. Confirm legend below lists all languages with matching colour dots and percentages
6. Confirm "Other" slice appears if top-6 languages don't sum to 100%
7. Confirm graceful empty state for accounts with no public repos

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Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors in changed file (npx tsc --noEmit 2>&1 | Select-String "LanguageBreakdown" → blank)
• npm run build → ✓ Compiled successfully
• Self-reviewed the diff
• Added/updated tests if applicable

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Accessibility Checklist

• Donut chart container has role="img" and aria-label
• Each legend dot SVG has role="img" and aria-label with language name
• Loading skeleton has role="status", aria-live="polite", aria-busy="true", and sr-only text
• Responsive UI verified

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Additional Notes

The LanguageBreakdown component, API route (/api/metrics/languages), widget ID (language-breakdown), and dashboard registration were all already in place. This PR upgrades only the visual layer — the data pipeline is unchanged.

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GSSoC Label Checklist 🏷️

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

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[mallya-m]

Hey @Priyanshu-byte-coder — PR is up!

What I built:

• Upgraded LanguageBreakdown.tsx from stacked bar → Recharts donut chart (single file change)
• Interactive tooltip on hover showing language name + percentage
• Centre label showing language count
• Expanded GitHub language color map (20 languages + Other fallback)
• Other bucket auto-calculated from percentage remainder
• Responsive: stacks on mobile, side-by-side on desktop
• Accessible: role="img" + aria-label on chart and legend dots

No new dependencies — recharts was already in the project. API route and dashboard registration were already in place.

Could you add level:intermediate and quality:clean labels to the PR when reviewing? Thanks!

Closes #938

[mallya-m]

Ready for review @Priyanshu-byte-coder! Single file change, no new dependencies, all CI checks passing.