Add CSV download for courses and registrations

[Copilot]

Adds the ability to download course and registration data as CSV files, triggered directly from the relevant UI sections.

Backend

• DownloadsController (new) — two endpoints:
  • GET /api/downloads/courses — all courses via ICourseService.SearchCoursesAsync(null, null)
  • GET /api/downloads/registrations — all registrations via new GetAllRegistrationsAsync()
• IRegistrationService / RegistrationService — added GetAllRegistrationsAsync() backed by the existing GetRegistrationsWithFiltersAsync(null, null, null) to bypass the 100-row pagination cap
• CSV escaping covers: commas, double-quotes, newlines, and formula injection (leading =, +, -, @ tab-prefixed)

Frontend

• Download CSV buttons added to the Courses and Registrations section action bars
• downloadCoursesCsv() / downloadRegistrationsCsv() — fetch blob, invoke browser download via a temporary anchor; errors surface through the existing showErrorModal
• Fallback filename uses Date.getUTC*() to match backend's DateTime.UtcNow convention

Tests

Nine unit tests added for DownloadsController using hand-written fakes (consistent with existing test style), covering: empty result sets, data rows, correct content type, comma quoting, double-quote escaping, and formula injection neutralisation.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add download feature</issue_title>
<issue_description>Implement a download feature that allows users to download resources or data as needed within the CourseApplication platform. This could include course materials, generated reports, or any files relevant to user activity. The new feature should integrate seamlessly with the current application structure and UI.

Acceptance Criteria:

• Users can trigger downloads from relevant sections.
• The implementation supports common file types (e.g., PDF, DOCX, CSV).
• Proper error handling is in place for failed downloads.
• Access permissions for downloads are respected based on user role.

Potential Enhancements:

• Download progress indicators for large files.
• Bulk/multi-file download capability.

Implementation notes and code pointers:

• The main front-end logic is located in frontend/script.js. This file handles navigation, UI updates, and contains candidate spots for adding UI controls and logic for download actions. For example, you can define new functions similar to refreshCourses, showCourseDetails, or utilize event handlers for modals to integrate download triggers (see script.js).
• For managing download UI (modals/buttons), also reference the modal-related styles in frontend/styles.css for consistent UI updates (see styles.css).
• To support backend file retrieval, consider creating an appropriate API endpoint in the .NET backend (possibly in Controllers/HomeController.cs) to serve downloadable files with access control (see HomeController.cs).
• Ensure error handling in downloads leverages the front-end's error modal functionality (see showErrorModal in script.js) and server-side HTTP error responses.

Please consider best practices for file security during the download process.</issue_description>

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

• Fixes Add download feature #65

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