refactor: enhance NewClient to support environment variable fallback and add tests

README.md

@@ -7,7 +7,11 @@ Go client library and CLI for the [Mail-in-a-Box](https://mailinabox.email/) adm
 ```go
 import miab "gitea.koszewscy.waw.pl/koszewscy/mailinabox-go"
 
-c := miab.NewClient("box.example.com", "admin@example.com", "password")
+// Explicit credentials:
+c, err := miab.NewClient("box.example.com", "admin@example.com", "password")
+
+// From environment variables (MIAB_* or MAILINABOX_* style):
+c, err := miab.NewClient("", "", "")
 
 c.SetRecord("foo.example.com", "TXT", "v=spf1 ~all")
 c.AddRecord("foo.example.com", "A", "1.2.3.4")
@@ -18,18 +22,30 @@ records, _ := c.ListRecords("TXT")
 
 ## CLI
 
-Install:
+Install from registry:
 
 ```sh
 go install gitea.koszewscy.waw.pl/koszewscy/mailinabox-go/cmd/miab@latest
 ```
 
-Credentials via environment variables:
+Build locally:
 
 ```sh
+go build -o miab ./cmd/miab
+```
+
+Credentials via environment variables (either style is accepted):
+
+```sh
+# MIAB style
 export MIAB_HOST=box.example.com
 export MIAB_USERNAME=admin@example.com
 export MIAB_PASSWORD=password
+
+# Mail-in-a-Box style
+export MAILINABOX_BASE_URL=https://box.example.com
+export MAILINABOX_EMAIL=admin@example.com
+export MAILINABOX_PASSWORD=password
 ```
 
 Commands: