feat(chat): add 2000 character limit for messages

Added message length validation to prevent spam and improve UX with
character counter feedback.

Backend:
- Added MESSAGE_MAX_LENGTH constant (2000 characters)
- Validation in send_event_message handler:
  - Check if content is string
  - Check if not empty after trim
  - Check if not exceeding max length
- Validation in send_match_message handler (same checks)
- Returns error message if validation fails

Frontend:
- Added MESSAGE_MAX_LENGTH constant (2000 characters)
- ChatInput component enhancements:
  - maxLength attribute on input (hard limit)
  - Character counter shows when >80% of limit
  - Counter turns red when at limit
  - Submit button disabled when at limit
  - Counter format: "X/2000"

UX:
- User sees counter at 1600+ characters (80% of limit)
- Hard limit prevents typing beyond 2000 chars
- Clear visual feedback (red text) when at limit
- Consistent validation on both event and match chats

Security:
- Prevents spam with extremely long messages
- Protects against potential DoS via message size
- Database already uses TEXT type (supports limit)

frontend/src/constants/index.js

@@ -5,3 +5,6 @@ export {
   CONNECTION_STATE,
   SUGGESTION_TYPE,
 } from './statuses';
+
+// Message validation
+export const MESSAGE_MAX_LENGTH = 2000;