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)

backend/src/constants/index.js

@@ -1,10 +1,14 @@
 const { MATCH_STATUS, SUGGESTION_STATUS } = require('./statuses');
 const { ACCOUNT_TIER, FAIRNESS_SUPPORTER_PENALTY, FAIRNESS_COMFORT_PENALTY } = require('./tiers');
 
+// Message validation
+const MESSAGE_MAX_LENGTH = 2000;
+
 module.exports = {
   MATCH_STATUS,
   SUGGESTION_STATUS,
   ACCOUNT_TIER,
   FAIRNESS_SUPPORTER_PENALTY,
   FAIRNESS_COMFORT_PENALTY,
+  MESSAGE_MAX_LENGTH,
 };