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.

database/db.go

@@ -6,7 +6,6 @@ import (
 	"bytes"
 	"errors"
 	"io"
-	"io/fs"
 	"log"
 	"os"
 	"path"
@@ -133,7 +132,7 @@ func isTableEmpty(tableName string) (bool, error) {
 // InitDB sets up the database connection, migrates models, and runs seeders.
 func InitDB(dbPath string) error {
 	dir := path.Dir(dbPath)
-	err := os.MkdirAll(dir, fs.ModePerm)
+	err := os.MkdirAll(dir, 0755)
 	if err != nil {
 		return err
 	}