fix: Validate server-only environment variables exclusively on the server to prevent browser validation errors.

lib/env.ts

@@ -58,8 +58,11 @@ export const envSchema = z
     const target = data.NEXT_PUBLIC_TARGET || data.TARGET;
     const isDev = target === 'development' || !target;
     const isBuildTimeValidation = process.env.SKIP_RUNTIME_ENV_VALIDATION === 'true';
+    const isServer = typeof window === 'undefined';
 
-    if (!isDev && !isBuildTimeValidation && !data.MAIL_HOST) {
+    // Only enforce server-only variables when running on the server.
+    // In the browser, non-NEXT_PUBLIC_ variables are undefined and should not trigger validation errors.
+    if (isServer && !isDev && !isBuildTimeValidation && !data.MAIL_HOST) {
       ctx.addIssue({
         code: z.ZodIssueCode.custom,
         message: 'MAIL_HOST is required in non-development environments',