Refactor project structure and update .gitignore; enhance README with setup instructions and environment requirements. Clean up backend code for improved readability and maintainability.

backend/src/main/java/com/sshmanager/entity/User.java

@@ -1,35 +1,35 @@
-package com.sshmanager.entity;
-
-import lombok.Data;
-import lombok.NoArgsConstructor;
-import lombok.AllArgsConstructor;
-
-import javax.persistence.*;
-import java.time.Instant;
-
-@Data
-@NoArgsConstructor
-@AllArgsConstructor
-@Entity
-@Table(name = "users")
-public class User {
-
-    @Id
-    @GeneratedValue(strategy = GenerationType.IDENTITY)
-    private Long id;
-
-    @Column(nullable = false, unique = true, length = 50)
-    private String username;
-
-    @Column(nullable = false, length = 255)
-    private String passwordHash;
-
-    @Column(length = 100)
-    private String displayName;
-
-    @Column(nullable = false)
-    private Instant createdAt = Instant.now();
-
-    @Column(nullable = false)
-    private Instant updatedAt = Instant.now();
-}
+package com.sshmanager.entity;
+
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import lombok.AllArgsConstructor;
+
+import javax.persistence.*;
+import java.time.Instant;
+
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+@Entity
+@Table(name = "users")
+public class User {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Column(nullable = false, unique = true, length = 50)
+    private String username;
+
+    @Column(nullable = false, length = 255)
+    private String passwordHash;
+
+    @Column(length = 100)
+    private String displayName;
+
+    @Column(nullable = false)
+    private Instant createdAt = Instant.now();
+
+    @Column(nullable = false)
+    private Instant updatedAt = Instant.now();
+}