feat: implement WYSIWYG markdown editor with Go backend and React frontend

- Backend: Go HTTP server with Cobra CLI (--data-dir, --port, --host flags)
- CRUD REST API for markdown files with JSON error responses
- File storage in flat directory structure (flat structure, .md files only)
- Comprehensive logrus logging for all operations
- Static file serving for frontend build (./frontend/dist)
- Frontend: React + TypeScript + Tailwind CSS
- Markdown editor with live GFM preview
- File management: list, create, open, save, delete
- Theme system (Dark, Light, System) with persistence
- Responsive design for desktop and mobile
- Backend tests (storage, API handlers) and frontend tests

internal/config/config.go

@@ -0,0 +1,54 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/spf13/cobra"
+)
+
+// Config holds the application configuration
+type Config struct {
+	DataDir string
+	Port    int
+	Host    string
+}
+
+// NewConfig creates a new config with default values
+func NewConfig() *Config {
+	return &Config{
+		DataDir: "./data",
+		Port:    8080,
+		Host:    "127.0.0.1",
+	}
+}
+
+// AddFlags adds the CLI flags to the command
+func (c *Config) AddFlags(cmd *cobra.Command) {
+	cmd.PersistentFlags().StringVar(&c.DataDir, "data-dir", c.DataDir, "Storage path for markdown files")
+	cmd.PersistentFlags().IntVar(&c.Port, "port", c.Port, "Server port")
+	cmd.PersistentFlags().StringVar(&c.Host, "host", c.Host, "Bind address")
+}
+
+// Validate validates the configuration
+func (c *Config) Validate() error {
+	if c.Port < 1 || c.Port > 65535 {
+		return fmt.Errorf("invalid port: %d", c.Port)
+	}
+	return nil
+}
+
+// DataDirPath returns the absolute path to the data directory
+func (c *Config) DataDirPath() string {
+	absPath, err := filepath.Abs(c.DataDir)
+	if err != nil {
+		return c.DataDir
+	}
+	return absPath
+}
+
+// EnsureDataDir ensures the data directory exists
+func (c *Config) EnsureDataDir() error {
+	return os.MkdirAll(c.DataDirPath(), 0755)
+}