Implement JSON-RPC client foundation

Add core JSON-RPC 2.0 protocol implementation for Kanboard API:

- JSONRPCRequest/Response/Error structs with proper JSON tags
- Generic call() method for sending requests and parsing responses
- Thread-safe request ID generation using atomic.Int64
- Automatic /jsonrpc.php path appending to baseURL
- Support for subdirectory installations
- HTTP Basic Auth support (API token and username/password)
- Error handling for unauthorized/forbidden responses

Includes comprehensive tests with httptest mock server.

Closes: kanboard-api-2g1

auth.go

@@ -0,0 +1,29 @@
+package kanboard
+
+import "net/http"
+
+// Authenticator applies authentication to HTTP requests.
+type Authenticator interface {
+	Apply(req *http.Request)
+}
+
+// apiTokenAuth implements API token authentication.
+type apiTokenAuth struct {
+	token string
+}
+
+// Apply adds HTTP Basic Auth with username "jsonrpc" and the API token.
+func (a *apiTokenAuth) Apply(req *http.Request) {
+	req.SetBasicAuth("jsonrpc", a.token)
+}
+
+// basicAuth implements username/password authentication.
+type basicAuth struct {
+	username string
+	password string
+}
+
+// Apply adds HTTP Basic Auth with username and password.
+func (a *basicAuth) Apply(req *http.Request) {
+	req.SetBasicAuth(a.username, a.password)
+}