feat: initialize warpbox.dev project structure and backend

Initialize the repository with the core Go backend architecture and a frontend mockup for warpbox.dev, a self-hosted file-sharing application.

- Set up Go backend modules for configuration, HTTP server, middleware, handlers, and templates.
- Add local development scripts, environment templates, and basic project configuration.
- Include a React-based frontend mockup under the docs directory.

backend/libs/handlers/upload.go

@@ -0,0 +1,19 @@
+package handlers
+
+import (
+	"net/http"
+
+	"warpbox.dev/backend/libs/helpers"
+)
+
+type uploadPlaceholderResponse struct {
+	Message       string `json:"message"`
+	MaxUploadSize string `json:"maxUploadSize"`
+}
+
+func (a *App) UploadPlaceholder(w http.ResponseWriter, r *http.Request) {
+	helpers.WriteJSON(w, http.StatusNotImplemented, uploadPlaceholderResponse{
+		Message:       "upload storage is not implemented yet; backend base is ready for the upload pipeline",
+		MaxUploadSize: a.uploadService.MaxUploadSizeLabel(),
+	})
+}