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/common/LoadingButton.jsx

@@ -0,0 +1,37 @@
+import { Loader2 } from 'lucide-react';
+
+/**
+ * Reusable Button component with built-in loading state
+ *
+ * @param {boolean} loading - Shows loading spinner when true
+ * @param {React.ReactNode} children - Button content when not loading
+ * @param {string} loadingText - Text to show when loading (default: "Loading...")
+ * @param {string} className - Additional CSS classes
+ * @param {object} ...props - All other button props (onClick, disabled, etc.)
+ */
+const LoadingButton = ({
+  loading = false,
+  children,
+  loadingText = 'Loading...',
+  className = '',
+  ...props
+}) => {
+  return (
+    <button
+      {...props}
+      disabled={loading || props.disabled}
+      className={className}
+    >
+      {loading ? (
+        <>
+          <Loader2 className="w-5 h-5 animate-spin inline mr-2" />
+          {loadingText}
+        </>
+      ) : (
+        children
+      )}
+    </button>
+  );
+};
+
+export default LoadingButton;