Admin panel is unintuitive and lacks user-friendly management + security risks #13

Problem:
The current Admin area is using basic scaffolding that is not only unintuitive but also poses security risks and is
functionally incomplete. It is currently "unusable" for a real-world moderator.

Key Issues identified:

1. Security Risk (XSS): The Description field accepts raw HTML input. This is a major security vulnerability
   (Cross-Site Scripting) and provides a terrible editing experience.
2. Broken UX (JSON Templates): CodeTemplatesJson requires the admin to manually type valid JSON into a textarea. This
   is highly error-prone and practically impossible to manage without mistakes.
3. Logic Flaw (Ordering): The OrderIndex (problem number) is manually entered by the user. This should be handled by
   the backend (auto-increment or logic-based) to ensure consistency.
4. Incomplete Lifecycle (Test Cases): The Test Case management is "write-only". Currently, an admin cannot view,
   modify, or delete existing tests once they are added. They can only upload new ones blindly.

Suggested Fixes:

• Sanitization: Replace raw HTML description with a Markdown editor or at least add server-side HTML sanitization.
• Template UI: Create dedicated tabs/text areas for each language (Python, Java, C++) in the Admin UI and have the
  backend bundle them into JSON automatically.
• Backend Ordering: Implement logic to automatically assign the next available OrderIndex.
• Full CRUD for Tests: Implement a proper interface to list, edit, and remove ProblemTests associated with a problem
  (ideally as a nested list inside the "Edit Problem" page).