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/router/router.go

@@ -0,0 +1,34 @@
+package router
+
+import (
+	"net/http"
+
+	"github.com/evanreichard/markdown-editor/internal/api"
+	"github.com/evanreichard/markdown-editor/internal/logging"
+)
+
+// New creates a new HTTP router with all routes configured
+func New(handlers *api.Handlers, staticDir string) *http.ServeMux {
+	mux := http.NewServeMux()
+
+	// API routes
+	mux.HandleFunc("GET /api/files", handlers.ListFiles)
+	mux.HandleFunc("GET /api/files/", handlers.GetFile)
+	mux.HandleFunc("POST /api/files", handlers.CreateFile)
+	mux.HandleFunc("PUT /api/files/", handlers.UpdateFile)
+	mux.HandleFunc("DELETE /api/files/", handlers.DeleteFile)
+
+	// Static assets
+	if staticDir != "" {
+		logging.Logger.WithField("static_dir", staticDir).Info("Static assets configured")
+		fs := http.FileServer(http.Dir(staticDir))
+		mux.Handle("/", fs)
+	} else {
+		mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			http.Error(w, "Static assets directory not configured", http.StatusNotFound)
+		})
+	}
+
+	logging.Logger.Info("Router initialized with all routes")
+	return mux
+}