Add CSV download feature for courses, students, and registrations

[Copilot]

Users have no way to export application data for offline use or reporting. This adds CSV download endpoints and corresponding UI buttons for courses, students, and registrations.

Backend

• New DownloadController with three endpoints:
  • GET /api/download/courses
  • GET /api/download/students
  • GET /api/download/registrations
• RFC-compliant CSV escaping handles commas, quotes, newlines, and carriage returns
• Filenames include datestamp (e.g. courses_20260320.csv)

Frontend

• Download CSV button added alongside Refresh in both the View Courses and Registrations sections
• downloadCsv(type) fetches the endpoint, resolves the filename from content-disposition, and triggers a browser file save with a safe setTimeout before revoking the object URL

async function downloadCsv(type) {
    const response = await fetch(`${API_BASE_URL}/download/${type}`);
    const blob = await response.blob();
    // filename extracted from content-disposition header
    const a = document.createElement('a');
    a.href = URL.createObjectURL(blob);
    a.download = filename;
    a.click();
    setTimeout(() => URL.revokeObjectURL(url), 100);
}

Tests

• 13 unit tests in DownloadControllerTests covering content-type, header rows, data values, CSV field escaping, and null-guard constructors (Moq added to test project)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add download feature to support file access</issue_title>
<issue_description>## Summary
Implement a download feature in the CourseApplication repository to allow users to download necessary files or data conveniently from the application.

Details

• Enable users to download course materials or reports directly from the application interface.
• Consider options to download different file formats (e.g., PDF, DOCX, CSV) if applicable.
• Ensure proper user permissions and security checks for downloads.
• Present a clear UI/UX for locating and initiating downloads.

Acceptance Criteria

• A "Download" button is available where appropriate.
• Files are downloaded reliably and securely.
• The feature is tested and documented.

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Please provide feedback if there are particular file types or download locations that should be prioritized.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add download feature to support file access #67

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