feat: Implement full network functionality with HTTP and P2P modes

This major update introduces a robust, multi-faceted networking
capability, allowing backups to be managed through a central server or
via direct peer-to-peer connections.

- **Add `serve` command**: Implemented a new `serve` command that runs a
persistent HTTP server. This allows for a centralized backup management
workflow where multiple clients can upload and download archives. -
**HTTP Client Mode**: The `create` and `restore` commands can now
function as HTTP clients, targeting a `serve` instance using the
`ip:port/filename` address format.

- **Restore P2P Mode**: The original peer-to-peer streaming
functionality (where `restore` acts as a TCP server and `create` as a
client) has been fully restored and integrated into the new, more robust
command architecture. - **Accurate Network Progress**: Fixed a critical
bug where network progress bars would not display correctly. The `serve`
command now sends a `Content-Length` header, and `restore` is refactored
to handle asynchronous state updates correctly. - **Smart Address
Parsing**: The application now automatically distinguishes between HTTP
mode (`ip:port/filename`) and P2P mode (`ip:port`), providing a seamless
user experience.

- **Update `README.md`**: The project's README has been completely
rewritten to document all three operating modes (Local, HTTP Server,
P2P) with clear usage examples. - **Add Gitea Actions Workflow**: A new
CI pipeline (`.gitea/workflows/build-and-test.yml`) has been added to
automatically build and test the application on every push and pull
request.

cmd/serve.go

@@ -0,0 +1,96 @@
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"github.com/gorilla/mux"
+	"github.com/spf13/cobra"
+)
+
+var (
+	serveAddress   string
+	serveDirectory string
+)
+
+var serveCmd = &cobra.Command{
+	Use:   "serve",
+	Short: "Run a backup server",
+	Long:  "Run an HTTP server to upload and download backups.",
+	Run: func(cmd *cobra.Command, args []string) {
+		if serveDirectory == "" {
+			home, err := os.UserHomeDir()
+			if err != nil {
+				log.Fatalf("Failed to get user home directory: %v", err)
+			}
+			serveDirectory = filepath.Join(home, "backups")
+		}
+
+		if err := os.MkdirAll(serveDirectory, 0755); err != nil {
+			log.Fatalf("Failed to create backup directory: %v", err)
+		}
+
+		r := mux.NewRouter()
+		r.HandleFunc("/backup/{filename}", uploadHandler).Methods("POST")
+		r.HandleFunc("/backup/{filename}", downloadHandler).Methods("GET")
+
+		log.Printf("Starting server on %s", serveAddress)
+		log.Printf("Using backup directory: %s", serveDirectory)
+		if err := http.ListenAndServe(serveAddress, r); err != nil {
+			log.Fatalf("Server failed: %v", err)
+		}
+	},
+}
+
+func uploadHandler(w http.ResponseWriter, r *http.Request) {
+	vars := mux.Vars(r)
+	filename := vars["filename"]
+	filePath := filepath.Join(serveDirectory, filename)
+
+	file, err := os.Create(filePath)
+	if err != nil {
+		http.Error(w, "Failed to create file", http.StatusInternalServerError)
+		log.Printf("Error creating file %s: %v", filename, err)
+		return
+	}
+	defer file.Close()
+
+	_, err = io.Copy(file, r.Body)
+	if err != nil {
+		http.Error(w, "Failed to write to file", http.StatusInternalServerError)
+		log.Printf("Error writing to file %s: %v", filename, err)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	fmt.Fprintf(w, "File %s uploaded successfully.", filename)
+	log.Printf("Uploaded %s", filename)
+}
+
+func downloadHandler(w http.ResponseWriter, r *http.Request) {
+	vars := mux.Vars(r)
+	filename := vars["filename"]
+	filePath := filepath.Join(serveDirectory, filename)
+
+	// Проверяем, существует ли файл, перед отправкой
+	if _, err := os.Stat(filePath); os.IsNotExist(err) {
+		http.NotFound(w, r)
+		log.Printf("File not found: %s", filename)
+		return
+	}
+
+	// http.ServeFile - это идиоматический способ отправки файлов в Go.
+	// Он автоматически устанавливает Content-Type, Content-Length и другие заголовки.
+	http.ServeFile(w, r, filePath)
+	log.Printf("Downloaded %s", filename)
+}
+
+func init() {
+	rootCmd.AddCommand(serveCmd)
+	serveCmd.Flags().StringVarP(&serveAddress, "address", "a", "localhost:8080", "Address and port for the server")
+	serveCmd.Flags().StringVarP(&serveDirectory, "directory", "d", "", "Directory to store backups (defaults to ~/backups)")
+}