Improve file upload validation and error handling in EduAid

Currently, the file upload component allows users to proceed without proper validation.
There is no feedback for unsupported file types, missing files, or oversized uploads.

This can confuse users and lead to failed uploads.

Expected behavior:
- Validate allowed file types (PDF, MP3)
- Show user-friendly error messages
- Disable upload action until input is valid
- Improve accessibility for error messages