feat: implement WYSIWYG markdown editor

Add complete markdown editor with Go backend and React/TypeScript frontend.

Backend:
- Cobra CLI with configurable host, port, data-dir, static-dir flags
- REST API for CRUD operations on markdown files (GET, POST, PUT, DELETE)
- File storage with flat .md structure
- Comprehensive Logrus logging for all operations
- Static asset serving for frontend

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Live markdown editor with GFM preview (react-markdown)
- File management UI (list, create, open, save, delete)
- Theme system (Light/Dark/System) with localStorage persistence
- Responsive design (320px - 1920px+)

Testing:
- 6 backend tests covering CRUD round-trip, validation, error handling
- 19 frontend tests covering API, theme system, and UI components
- All tests passing with single 'make test' command

Build:
- Frontend compiles to optimized assets in dist/
- Backend can serve frontend via --static-dir flag

backend/internal/config/config.go

@@ -0,0 +1,24 @@
+package config
+
+import "fmt"
+
+// Config holds the server configuration
+type Config struct {
+	DataDir string
+	Port    string
+	Host    string
+}
+
+// Default returns the default configuration
+func Default() *Config {
+	return &Config{
+		DataDir: "./data",
+		Port:    "8080",
+		Host:    "127.0.0.1",
+	}
+}
+
+// Addr returns the full address to bind to
+func (c *Config) Addr() string {
+	return fmt.Sprintf("%s:%s", c.Host, c.Port)
+}