Add WithAuthHeader for custom authentication header name

Support configuring a custom HTTP header name for authentication
(e.g., X-API-Auth instead of Authorization) for Kanboard servers
with specific proxy configurations that use alternative auth headers.

- Add headerName field to apiTokenAuth and basicAuth structs
- Add WithAuthHeader() fluent method to Client
- Auth methods pass custom header name when creating auth structs
- Add tests for custom header with API token, basic auth, and custom user

Closes kanboard-9wa

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

auth.go

@@ -1,6 +1,9 @@
 package kanboard
 
-import "net/http"
+import (
+	"encoding/base64"
+	"net/http"
+)
 
 // Authenticator applies authentication to HTTP requests.
 type Authenticator interface {
@@ -9,8 +12,9 @@ type Authenticator interface {
 
 // apiTokenAuth implements API token authentication.
 type apiTokenAuth struct {
-	user  string
-	token string
+	user       string
+	token      string
+	headerName string
 }
 
 // Apply adds HTTP Basic Auth with the configured user (or "jsonrpc" if empty) and the API token.
@@ -19,16 +23,31 @@ func (a *apiTokenAuth) Apply(req *http.Request) {
 	if user == "" {
 		user = "jsonrpc"
 	}
-	req.SetBasicAuth(user, a.token)
+	if a.headerName != "" {
+		req.Header.Set(a.headerName, "Basic "+basicAuthValue(user, a.token))
+	} else {
+		req.SetBasicAuth(user, a.token)
+	}
 }
 
 // basicAuth implements username/password authentication.
 type basicAuth struct {
-	username string
-	password string
+	username   string
+	password   string
+	headerName string
 }
 
 // Apply adds HTTP Basic Auth with username and password.
 func (a *basicAuth) Apply(req *http.Request) {
-	req.SetBasicAuth(a.username, a.password)
+	if a.headerName != "" {
+		req.Header.Set(a.headerName, "Basic "+basicAuthValue(a.username, a.password))
+	} else {
+		req.SetBasicAuth(a.username, a.password)
+	}
+}
+
+// basicAuthValue returns the base64-encoded value for HTTP Basic Auth.
+func basicAuthValue(username, password string) string {
+	auth := username + ":" + password
+	return base64.StdEncoding.EncodeToString([]byte(auth))
 }