Initial commit: WYSIWYG Markdown Editor - Go backend + React/TypeScript frontend with Tailwind CSS

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- Gin HTTP server with REST API for markdown CRUD operations
- File storage on disk (.md files only)
- Comprehensive logrus logging
- Backend tests with CRUD round-trip verification

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Markdown editor with live GFM preview (react-markdown + remark-gfm)
- File management UI (list, create, open, save, delete)
- Theme switcher with Dark/Light/System modes
- Responsive design
- Frontend tests with vitest

Testing:
- All backend tests pass (go test ./...)
- All frontend tests pass (npm test)

internal/api/api.go

@@ -0,0 +1,149 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/sirupsen/logrus"
+	"markdown-editor/internal/storage"
+	"markdown-editor/internal/logger"
+
+	"github.com/gin-gonic/gin"
+)
+
+type API struct {
+	storage *storage.Storage
+	log     *logrus.Logger
+}
+
+func NewAPI(storage *storage.Storage) *API {
+	log := logger.GetLogger()
+	return &API{
+		storage: storage,
+		log:     log,
+	}
+}
+
+// FileInfo represents a file for API responses
+type FileInfo struct {
+	Name    string `json:"name"`
+	Content string `json:"content,omitempty"`
+}
+
+// File represents a file for API requests
+type File struct {
+	Name    string `json:"name"`
+	Content string `json:"content"`
+}
+
+// ErrorResponse represents an error response
+type ErrorResponse struct {
+	Error string `json:"error"`
+}
+
+func (a *API) RegisterRoutes(router *gin.Engine) {
+	// File endpoints
+	router.GET("/api/files", a.listFiles)
+	router.GET("/api/files/:name", a.getFile)
+	router.POST("/api/files", a.createFile)
+	router.PUT("/api/files/:name", a.updateFile)
+	router.DELETE("/api/files/:name", a.deleteFile)
+}
+
+func (a *API) listFiles(c *gin.Context) {
+	a.log.Info("Handling GET /api/files")
+
+	files, err := a.storage.ListFiles()
+	if err != nil {
+		a.log.Errorf("Failed to list files: %v", err)
+		c.JSON(http.StatusInternalServerError, ErrorResponse{Error: "Failed to list files"})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"files": files,
+	})
+}
+
+func (a *API) getFile(c *gin.Context) {
+	name := c.Param("name")
+	a.log.WithField("filename", name).Info("Handling GET /api/files/:name")
+
+	file, err := a.storage.GetFile(name)
+	if err != nil {
+		a.log.Errorf("Failed to get file %s: %v", name, err)
+		if err.Error() == "file not found: "+name {
+			c.JSON(http.StatusNotFound, ErrorResponse{Error: "File not found"})
+			return
+		}
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, file)
+}
+
+func (a *API) createFile(c *gin.Context) {
+	var file File
+	if err := c.ShouldBindJSON(&file); err != nil {
+		a.log.Errorf("Invalid request body: %v", err)
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: "Invalid request body"})
+		return
+	}
+
+	a.log.WithField("filename", file.Name).Info("Handling POST /api/files")
+
+	if file.Name == "" {
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: "File name is required"})
+		return
+	}
+
+	if err := a.storage.CreateFile(file.Name, file.Content); err != nil {
+		a.log.Errorf("Failed to create file %s: %v", file.Name, err)
+		if err.Error() == "file already exists: "+file.Name {
+			c.JSON(http.StatusConflict, ErrorResponse{Error: "File already exists"})
+			return
+		}
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusCreated, file)
+}
+
+func (a *API) updateFile(c *gin.Context) {
+	name := c.Param("name")
+	var file File
+	if err := c.ShouldBindJSON(&file); err != nil {
+		a.log.Errorf("Invalid request body: %v", err)
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: "Invalid request body"})
+		return
+	}
+
+	a.log.WithField("filename", name).Info("Handling PUT /api/files/:name")
+
+	if file.Name != name {
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: "File name in path and body must match"})
+		return
+	}
+
+	if err := a.storage.UpdateFile(name, file.Content); err != nil {
+		a.log.Errorf("Failed to update file %s: %v", name, err)
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, file)
+}
+
+func (a *API) deleteFile(c *gin.Context) {
+	name := c.Param("name")
+	a.log.WithField("filename", name).Info("Handling DELETE /api/files/:name")
+
+	if err := a.storage.DeleteFile(name); err != nil {
+		a.log.Errorf("Failed to delete file %s: %v", name, err)
+		c.JSON(http.StatusBadRequest, ErrorResponse{Error: err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusNoContent, nil)
+}