fix: update Tailwind CSS import syntax for v4

- Change from @tailwind directives to @import syntax
- Move shimmer keyframes to CSS file (v4 convention)
- Fix Tailwind classes not being processed

src/styles/tailwind.css

@@ -1,4 +1,7 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
+
+@keyframes shimmer {
+  0% { transform: translateX(-100%); }
+  100% { transform: translateX(100%); }
+}