Move constants, add API URL, and implement file URL helper

Relocated common constants to a new "lib/constants" directory for better organization. Added BACKEND_API_URL constant to manage API base URL dynamically. Introduced a utility function `getServerFileUrl` to generate file URLs using the backend API.

frontend/src/lib/constants/common.ts

@@ -1 +1,2 @@
 export const RESERVED_SLUGS = ["new", "edit", "delete", "settings"];
+export const BACKEND_API_URL = process.env.NEXT_PUBLIC_API_URL;

frontend/src/lib/utils.ts

@@ -1,5 +1,6 @@
 import { clsx, type ClassValue } from "clsx";
 import { twMerge } from "tailwind-merge";
+import { BACKEND_API_URL } from "@/lib/constants";
 
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs));
@@ -65,3 +66,7 @@ export function formatTime(
     minute: "2-digit",
   });
 }
+
+export const getServerFileUrl = (url?: string) => {
+  return url ? `${BACKEND_API_URL}/files/${url}` : undefined;
+};