refactor(env): update NEXT_PUBLIC_API_URL to use 'auto' for better flexibility

frontend/src/api/mutator.ts

@@ -1,4 +1,5 @@
 import { getLocalAuthToken, isLocalAuthMode } from "@/auth/localAuth";
+import { getApiBaseUrl } from "@/lib/api-base";
 
 type ClerkSession = {
   getToken: () => Promise<string>;
@@ -39,11 +40,7 @@ export const customFetch = async <T>(
   url: string,
   options: RequestInit,
 ): Promise<T> => {
-  const rawBaseUrl = process.env.NEXT_PUBLIC_API_URL;
-  if (!rawBaseUrl) {
-    throw new Error("NEXT_PUBLIC_API_URL is not set.");
-  }
-  const baseUrl = rawBaseUrl.replace(/\/+$/, "");
+  const baseUrl = getApiBaseUrl();
 
   const headers = new Headers(options.headers);
   const hasBody = options.body !== undefined && options.body !== null;

frontend/src/components/organisms/LocalAuthLogin.tsx

@@ -7,17 +7,18 @@ import { setLocalAuthToken } from "@/auth/localAuth";
 import { Button } from "@/components/ui/button";
 import { Card, CardContent, CardHeader } from "@/components/ui/card";
 import { Input } from "@/components/ui/input";
+import { getApiBaseUrl } from "@/lib/api-base";
 
 const LOCAL_AUTH_TOKEN_MIN_LENGTH = 50;
 
 async function validateLocalToken(token: string): Promise<string | null> {
-  const rawBaseUrl = process.env.NEXT_PUBLIC_API_URL;
-  if (!rawBaseUrl) {
-    return "NEXT_PUBLIC_API_URL is not set.";
+  let baseUrl: string;
+  try {
+    baseUrl = getApiBaseUrl();
+  } catch {
+    return "Unable to resolve backend URL.";
   }
 
-  const baseUrl = rawBaseUrl.replace(/\/+$/, "");
-
   let response: Response;
   try {
     response = await fetch(`${baseUrl}/api/v1/users/me`, {

frontend/src/lib/api-base.test.ts

@@ -0,0 +1,27 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { getApiBaseUrl } from "./api-base";
+
+describe("getApiBaseUrl", () => {
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
+  it("returns normalized explicit URL", () => {
+    vi.stubEnv("NEXT_PUBLIC_API_URL", "https://api.example.com///");
+
+    expect(getApiBaseUrl()).toBe("https://api.example.com");
+  });
+
+  it("auto-resolves from browser host when set to auto", () => {
+    vi.stubEnv("NEXT_PUBLIC_API_URL", "auto");
+
+    expect(getApiBaseUrl()).toBe("http://localhost:8000");
+  });
+
+  it("auto-resolves from browser host when unset", () => {
+    vi.stubEnv("NEXT_PUBLIC_API_URL", "");
+
+    expect(getApiBaseUrl()).toBe("http://localhost:8000");
+  });
+});

frontend/src/lib/api-base.ts

@@ -1,11 +1,22 @@
 export function getApiBaseUrl(): string {
-  const raw = process.env.NEXT_PUBLIC_API_URL;
-  if (!raw) {
-    throw new Error("NEXT_PUBLIC_API_URL is not set.");
+  const raw = process.env.NEXT_PUBLIC_API_URL?.trim();
+  if (raw && raw.toLowerCase() !== "auto") {
+    const normalized = raw.replace(/\/+$/, "");
+    if (!normalized) {
+      throw new Error("NEXT_PUBLIC_API_URL is invalid.");
+    }
+    return normalized;
   }
-  const normalized = raw.replace(/\/+$/, "");
-  if (!normalized) {
-    throw new Error("NEXT_PUBLIC_API_URL is invalid.");
+
+  if (typeof window !== "undefined") {
+    const protocol = window.location.protocol === "https:" ? "https" : "http";
+    const host = window.location.hostname;
+    if (host) {
+      return `${protocol}://${host}:8000`;
+    }
   }
-  return normalized;
+
+  throw new Error(
+    "NEXT_PUBLIC_API_URL is not set and cannot be auto-resolved outside the browser.",
+  );
 }