feat(frontend): create reusable components for Phase 1 refactoring

- Add Alert component with 4 variants (success/error/warning/info)
- Add LoadingSpinner and LoadingButton components
- Add FormInput and FormSelect components with icon support
- Add Modal and ConfirmationModal components
- Add ChatMessage, ChatMessageList, and ChatInput components
- Add EventCard component

These components will eliminate ~450 lines of duplicated code across pages.
Part of Phase 1 (Quick Wins) frontend refactoring.

frontend/src/components/chat/ChatInput.jsx

@@ -0,0 +1,52 @@
+import { Send } from 'lucide-react';
+
+/**
+ * Chat Input component with send button
+ *
+ * @param {string} value - Input value
+ * @param {function} onChange - Change handler for input
+ * @param {function} onSubmit - Submit handler for form
+ * @param {boolean} disabled - Whether input is disabled (e.g., when not connected)
+ * @param {string} placeholder - Placeholder text (default: "Write a message...")
+ * @param {string} className - Additional classes for the form
+ * @param {boolean} autoFocus - Whether to autofocus the input
+ */
+const ChatInput = ({
+  value,
+  onChange,
+  onSubmit,
+  disabled = false,
+  placeholder = 'Write a message...',
+  className = '',
+  autoFocus = false
+}) => {
+  const handleSubmit = (e) => {
+    e.preventDefault();
+    if (!value.trim() || disabled) return;
+    onSubmit(e);
+  };
+
+  return (
+    <form onSubmit={handleSubmit} className={`flex space-x-2 ${className}`}>
+      <input
+        type="text"
+        value={value}
+        onChange={onChange}
+        placeholder={placeholder}
+        disabled={disabled}
+        autoFocus={autoFocus}
+        className="flex-1 px-4 py-2 border border-gray-300 rounded-lg focus:ring-primary-500 focus:border-primary-500 disabled:opacity-50 disabled:cursor-not-allowed"
+      />
+      <button
+        type="submit"
+        disabled={disabled || !value.trim()}
+        className="px-4 py-2 bg-primary-600 text-white rounded-lg hover:bg-primary-700 transition-colors disabled:opacity-50 disabled:cursor-not-allowed"
+        aria-label="Send message"
+      >
+        <Send className="w-5 h-5" />
+      </button>
+    </form>
+  );
+};
+
+export default ChatInput;