feat: create globals.css for Next.js app

frontend/src/app/globals.css

@@ -0,0 +1,23 @@
+/* Global styles for the Next.js app */
+html,
+body {
+  padding: 0;
+  margin: 0;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, 'Noto Sans',
+    sans-serif;
+  background-color: #f9fafb; /* light background */
+  color: #111827; /* dark text */
+}
+
+*,
+*::before,
+*::after {
+  box-sizing: border-box;
+}
+
+/* Center content by default */
+.container {
+  max-width: 1200px;
+  margin: 0 auto;
+  padding: 0 1rem;
+}