feat: first version of the interface, integrate Tailwind CSS and update routing

- Add Tailwind CSS dependencies and configure Vite to use Tailwind
- Implement routing with React Router for Landing and Dashboard pages
- Remove unused App.css file and refactor App component to utilize new structure
- Update global styles in index.css to incorporate Tailwind's utility classes

frontend/src/components/ui/Spinner.jsx

@@ -0,0 +1,26 @@
+/**
+ * Small inline spinner. Uses Tailwind's `animate-spin` utility, so no custom
+ * keyframes are required. Inherits colour from its parent via `currentColor`.
+ */
+export function Spinner({ size = 16, className = "" }) {
+  return (
+    <svg
+      width={size}
+      height={size}
+      viewBox="0 0 24 24"
+      fill="none"
+      className={`animate-spin ${className}`}
+      aria-hidden="true"
+    >
+      <circle
+        cx="12"
+        cy="12"
+        r="10"
+        stroke="currentColor"
+        strokeWidth="3"
+        strokeDasharray="40 20"
+        strokeLinecap="round"
+      />
+    </svg>
+  );
+}