Add centralized error messages and validation utils

Introduce errorMessages.js with i18n translation keys for all
user-facing errors (form, connection, API) and validators.js with
reusable validation functions that reference those keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/utils/validators.js

@@ -0,0 +1,25 @@
+import { ERROR_MESSAGES } from '../constants/errorMessages'
+
+export function validateRequired(value, message) {
+  if (!value || !String(value).trim()) return message
+  return null
+}
+
+export function validateJoinForm(username, room) {
+  if (!username.trim() || !room.trim()) {
+    return ERROR_MESSAGES.form.usernameAndRoomRequired
+  }
+  return null
+}
+
+export function validateUsername(username) {
+  return validateRequired(username, ERROR_MESSAGES.form.usernameRequired)
+}
+
+export function validateRoom(room) {
+  return validateRequired(room, ERROR_MESSAGES.form.roomRequired)
+}
+
+export function validateMessageText(text) {
+  return validateRequired(text, ERROR_MESSAGES.form.messageTextRequired)
+}