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

errors.go

@@ -0,0 +1,28 @@
+package kanboard
+
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	// ErrConnectionFailed indicates a connection to the Kanboard server failed.
+	ErrConnectionFailed = errors.New("connection to Kanboard server failed")
+
+	// ErrUnauthorized indicates authentication failed.
+	ErrUnauthorized = errors.New("authentication failed: invalid credentials")
+
+	// ErrForbidden indicates insufficient permissions.
+	ErrForbidden = errors.New("access forbidden: insufficient permissions")
+)
+
+// APIError represents an error returned by the Kanboard API.
+type APIError struct {
+	Code    int
+	Message string
+}
+
+// Error implements the error interface.
+func (e *APIError) Error() string {
+	return fmt.Sprintf("Kanboard API error (code %d): %s", e.Code, e.Message)
+}