feat: add pie chart toggle to LanguageBreakdown widget

[WhyAvya]

Summary

Closes #224

The existing LanguageBreakdown widget only showed a stacked bar chart.
This PR adds a pie chart view with a Pie/Bar toggle button, as mentioned
in the issue (pie/bar chart requirement).

Type of Change

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

Changes Made

• Added pie chart using Recharts PieChart component
• Added Pie/Bar toggle buttons in the widget header
• Pie chart is set as the default view
• Bar chart kept as alternative view

How to Test

Steps for the reviewer to verify this works:

1. Sign in to the dashboard
2. Scroll down to Language Breakdown widget
3. Click Pie button to see pie chart
4. Click Bar button to see stacked bar chart

Screenshots

Pie Chart View (Default)

New pie chart view showing language distribution with Pie/Bar toggle buttons

Bar Chart View

Original stacked bar chart view — still accessible via toggle

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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