fix(layout): restore mx-auto centering by scoping CSS reset to @layer base

Unlayered CSS always wins over Tailwind's @layer utilities, so the
bare * { margin: 0 } reset was overriding mx-auto everywhere. Moving
it into @layer base restores correct cascade order.

src/app/globals.css

@@ -39,10 +39,12 @@ html {
   font-family: var(--font-mono);
 }
 
-* {
-  box-sizing: border-box;
-  margin: 0;
-  padding: 0;
+@layer base {
+  * {
+    box-sizing: border-box;
+    margin: 0;
+    padding: 0;
+  }
 }
 
 /* Default transitions — linear, fast */