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/components/Header.tsx

@@ -7,7 +7,17 @@ import constants from "../../../shared/constants.json";
 
 const { ADMIN, REGULAR } = constants.roles;
 
-type PageId = "counter" | "sum" | "profile" | "invites" | "booking" | "appointments" | "audit" | "admin-invites" | "admin-availability" | "admin-appointments";
+type PageId =
+  | "counter"
+  | "sum"
+  | "profile"
+  | "invites"
+  | "booking"
+  | "appointments"
+  | "audit"
+  | "admin-invites"
+  | "admin-availability"
+  | "admin-appointments";
 
 interface HeaderProps {
   currentPage: PageId;
@@ -82,4 +92,3 @@ export function Header({ currentPage }: HeaderProps) {
     </div>
   );
 }
-