Add Prettier for TypeScript formatting

- Install prettier
- Configure .prettierrc.json and .prettierignore
- Add npm scripts: format, format:check
- Add Makefile target: format-frontend
- Format all frontend files

frontend/app/signup/[code]/page.tsx

@@ -13,7 +13,7 @@ export default function SignupWithCodePage() {
   useEffect(() => {
     // Wait for auth check to complete before redirecting
     if (isLoading) return;
-    
+
     if (user) {
       // Already logged in, redirect to home
       router.replace("/");
@@ -25,15 +25,17 @@ export default function SignupWithCodePage() {
   }, [user, isLoading, code, router]);
 
   return (
-    <main style={{ 
-      minHeight: "100vh", 
-      display: "flex", 
-      alignItems: "center", 
-      justifyContent: "center",
-      background: "linear-gradient(135deg, #0f0f23 0%, #1a1a3e 50%, #0f0f23 100%)",
-      color: "rgba(255,255,255,0.6)",
-      fontFamily: "'DM Sans', system-ui, sans-serif",
-    }}>
+    <main
+      style={{
+        minHeight: "100vh",
+        display: "flex",
+        alignItems: "center",
+        justifyContent: "center",
+        background: "linear-gradient(135deg, #0f0f23 0%, #1a1a3e 50%, #0f0f23 100%)",
+        color: "rgba(255,255,255,0.6)",
+        fontFamily: "'DM Sans', system-ui, sans-serif",
+      }}
+    >
       Redirecting...
     </main>
   );