feat(frontend): implement data export (CSV/JSON) for analytics dashboards, credentials, and progress reports

[Emmanuel-Ugochukwu1]

closes #184

• Created frontend/src/utils/dataExport.ts: generic CSV/JSON export utility with CSV escaping, pretty-printed JSON, dated filenames, and Blob download
• Created frontend/src/utils/tests/dataExport.test.ts: 24 unit tests covering escapeCSVField, convertToCSV, convertToJSON, download, and edge cases
• Updated frontend/src/components/Analytics/ExportButton.tsx: reusable export button with CSV/JSON format dropdown, uses new dataExport utility
• Added export to frontend/src/components/CredentialList.tsx: exports all credentials as CSV/JSON with headers, dates, and skills
• Added export to frontend/src/components/dashboard/ProgressDashboard.tsx: exports enrollment and learning activity data, tracks active tab
• Added per-chart export to frontend/src/components/Analytics/DashboardCharts.tsx: each chart (User Growth, Enrollments, Credentials) has its own export dropdown

Description

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Fixes #31

Type of Change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas