refactor: consolidate CSS files by removing unused styles and ensuring consistent height settings

Author: sumingcheng

src/index.css

@@ -5,3 +5,7 @@
 :root {
   font-family: Inter, system-ui, Avenir, Helvetica, Arial, sans-serif;
 }
+
+html, body, #root {
+  height: 100%;
+}