feat: support subpath deployment with NEXT_PUBLIC_BASE_PATH environment variable

next.config.ts

@@ -18,15 +18,28 @@ try {
   // VERSION file not found
 }
 
+// Subpath deployment, e.g. NEXT_PUBLIC_BASE_PATH=/webmail. Read at build time
+// because Next.js bakes basePath into emitted asset URLs and route metadata.
+// Trailing slash is stripped; an empty/missing value disables the feature.
+const rawBasePath = process.env.NEXT_PUBLIC_BASE_PATH?.trim() ?? "";
+const basePath = rawBasePath.replace(/\/+$/, "");
+if (basePath && !basePath.startsWith("/")) {
+  throw new Error(
+    `NEXT_PUBLIC_BASE_PATH must start with "/" (got: ${JSON.stringify(rawBasePath)})`
+  );
+}
+
 const nextConfig: NextConfig = {
   output: "standalone",
   allowedDevOrigins: ["192.168.1.51"],
+  basePath: basePath || undefined,
   turbopack: {
     root: import.meta.dirname,
   },
   env: {
     NEXT_PUBLIC_GIT_COMMIT: gitCommitHash,
     NEXT_PUBLIC_APP_VERSION: appVersion,
+    NEXT_PUBLIC_BASE_PATH: basePath,
   },
 };