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

client.go

@@ -0,0 +1,47 @@
+package kanboard
+
+import (
+	"net/http"
+	"strings"
+)
+
+// Client is the Kanboard API client.
+type Client struct {
+	baseURL    string
+	endpoint   string
+	httpClient *http.Client
+	auth       Authenticator
+}
+
+// NewClient creates a new Kanboard API client.
+// The baseURL should be the base URL of the Kanboard instance (e.g., "https://kanboard.example.com").
+// The path /jsonrpc.php is appended automatically.
+// Supports subdirectory installations (e.g., "https://example.com/kanboard" → POST https://example.com/kanboard/jsonrpc.php).
+func NewClient(baseURL string) *Client {
+	// Ensure no trailing slash
+	baseURL = strings.TrimSuffix(baseURL, "/")
+
+	return &Client{
+		baseURL:    baseURL,
+		endpoint:   baseURL + "/jsonrpc.php",
+		httpClient: http.DefaultClient,
+	}
+}
+
+// WithAPIToken configures the client to use API token authentication.
+func (c *Client) WithAPIToken(token string) *Client {
+	c.auth = &apiTokenAuth{token: token}
+	return c
+}
+
+// WithBasicAuth configures the client to use username/password authentication.
+func (c *Client) WithBasicAuth(username, password string) *Client {
+	c.auth = &basicAuth{username: username, password: password}
+	return c
+}
+
+// WithHTTPClient sets a custom HTTP client.
+func (c *Client) WithHTTPClient(client *http.Client) *Client {
+	c.httpClient = client
+	return c
+}