Fix overly permissive file permissions (os.ModePerm) (#4207)

Several file operations used os.ModePerm (0777) which makes files
world-writable and world-readable, violating the principle of least
privilege:

- database/db.go: InitDB directory creation → 0755
- xray/process.go: Xray config write → 0644
- xray/process.go: Crash report write → 0644
- web/service/server.go: Binary extraction → 0755

Also removes unused "io/fs" imports from the affected files.

web/service/server.go

@@ -7,7 +7,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"io/fs"
 	"mime/multipart"
 	"net/http"
 	"os"
@@ -660,7 +659,7 @@ func (s *ServerService) UpdateXray(version string) error {
 		defer zipFile.Close()
 		os.MkdirAll(filepath.Dir(fileName), 0755)
 		os.Remove(fileName)
-		file, err := os.OpenFile(fileName, os.O_CREATE|os.O_RDWR|os.O_TRUNC, fs.ModePerm)
+		file, err := os.OpenFile(fileName, os.O_CREATE|os.O_RDWR|os.O_TRUNC, 0755)
 		if err != nil {
 			return err
 		}