fix: add validation to reject invalid characters in assignment ID

[Gasg10]

Fixes #98

Problem

When creating an assignment with a backslash (\) in the ID field, the application returned an HTTP 404 error instead of a proper validation message.

Root Cause

The assignmentId field in AssignmentForm.kt had no pattern validation, so any character was accepted. When a backslash was submitted, the URL routing failed with 404.

Fix

Added a @Pattern annotation to the assignmentId field in AssignmentForm.kt to only allow letters, numbers, hyphens and underscores:

@field:NotEmpty(message = "Error: Assignment Id must not be empty")
@field:Pattern(regexp = "^[a-zA-Z0-9_-]+$", message = "Error: Assignment Id must only contain letters, numbers, hyphens and underscores")
var assignmentId: String? = null,

Testing

1. Start the application locally
2. Log in as teacher
3. Navigate to Create Assignment
4. Enter test\assignment in the ID field and fill the other required fields
5. Click Save — a clear validation error message is now shown instead of a 404 error