Implement RBAC with login screen for admin and staff roles

- Add Role enum (ADMIN, STAFF) and UserSession singleton
- Add User model and UserDB with SHA2-256 authentication
- Add login screen (login-view.fxml + LoginController)
- Update main-layout-view.fxml: dynamic username/role labels, logout button
- Update MainLayoutController: hide Inventory/Suppliers/Product-Suppliers for STAFF, show username/role, handle logout
- Update PetShopApplication to start with login screen and initialize users table
- Update module-info.java to open/export auth package

src/main/resources/org/example/petshopdesktop/login-view.fxml

@@ -0,0 +1,80 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<?import javafx.geometry.Insets?>
+<?import javafx.scene.control.Button?>
+<?import javafx.scene.control.Label?>
+<?import javafx.scene.control.PasswordField?>
+<?import javafx.scene.control.TextField?>
+<?import javafx.scene.layout.VBox?>
+<?import javafx.scene.text.Font?>
+
+<VBox alignment="CENTER" prefHeight="400.0" prefWidth="380.0" spacing="16.0"
+      style="-fx-background-color: #2C3E50;"
+      xmlns="http://javafx.com/javafx/25"
+      xmlns:fx="http://javafx.com/fxml/1"
+      fx:controller="org.example.petshopdesktop.controllers.LoginController">
+   <padding>
+      <Insets bottom="40.0" left="50.0" right="50.0" top="40.0" />
+   </padding>
+   <children>
+      <Label text="🐾 Pet Shop Manager" textFill="WHITE">
+         <font>
+            <Font name="Comic Sans MS Bold" size="22.0" />
+         </font>
+         <VBox.margin>
+            <Insets bottom="10.0" />
+         </VBox.margin>
+      </Label>
+
+      <Label text="Username" textFill="#cccccc">
+         <font>
+            <Font name="System Bold" size="13.0" />
+         </font>
+      </Label>
+      <TextField fx:id="txtUsername" promptText="Enter username"
+                 style="-fx-background-color: #3d5166; -fx-text-fill: white; -fx-prompt-text-fill: #888; -fx-background-radius: 8; -fx-border-width: 0;">
+         <padding>
+            <Insets bottom="10.0" left="12.0" right="12.0" top="10.0" />
+         </padding>
+         <font>
+            <Font size="14.0" />
+         </font>
+      </TextField>
+
+      <Label text="Password" textFill="#cccccc">
+         <font>
+            <Font name="System Bold" size="13.0" />
+         </font>
+      </Label>
+      <PasswordField fx:id="txtPassword" promptText="Enter password"
+                    style="-fx-background-color: #3d5166; -fx-text-fill: white; -fx-prompt-text-fill: #888; -fx-background-radius: 8; -fx-border-width: 0;">
+         <padding>
+            <Insets bottom="10.0" left="12.0" right="12.0" top="10.0" />
+         </padding>
+         <font>
+            <Font size="14.0" />
+         </font>
+      </PasswordField>
+
+      <Label fx:id="lblError" text="" textFill="#FF6B6B" wrapText="true">
+         <font>
+            <Font size="13.0" />
+         </font>
+      </Label>
+
+      <Button fx:id="btnLogin" mnemonicParsing="false" onAction="#btnLoginClicked"
+              prefWidth="280.0"
+              style="-fx-background-color: #FF6B6B; -fx-background-radius: 8; -fx-cursor: hand;"
+              text="Login" textFill="WHITE">
+         <font>
+            <Font name="System Bold" size="15.0" />
+         </font>
+         <padding>
+            <Insets bottom="12.0" left="12.0" right="12.0" top="12.0" />
+         </padding>
+         <VBox.margin>
+            <Insets top="8.0" />
+         </VBox.margin>
+      </Button>
+   </children>
+</VBox>

src/main/resources/org/example/petshopdesktop/main-layout-view.fxml

@@ -16,12 +16,12 @@
             <Insets bottom="30.0" left="30.0" right="30.0" top="30.0" />
          </padding>
          <children>
-            <Label text="Name" textFill="WHITE">
+            <Label fx:id="lblUsername" text="Name" textFill="WHITE">
                <font>
                   <Font name="Comic Sans MS Bold" size="30.0" />
                </font>
             </Label>
-            <Label text="Pet Store Manager" textFill="#ffe66d">
+            <Label fx:id="lblRole" text="Pet Store Manager" textFill="#ffe66d">
                <font>
                   <Font name="Comic Sans MS Bold" size="16.0" />
                </font>
@@ -106,6 +106,17 @@
                   <Insets bottom="12.0" left="12.0" right="45.0" top="12.0" />
                </padding>
             </Button>
+            <Button fx:id="btnLogout" mnemonicParsing="false" onAction="#btnLogoutClicked" prefWidth="250.0" style="-fx-background-color: #34495E; -fx-background-radius: 8; -fx-cursor: hand;" text="🚪 Logout" textFill="#cccccc">
+               <font>
+                  <Font name="Comic Sans MS Bold" size="14.0" />
+               </font>
+               <padding>
+                  <Insets bottom="12.0" left="12.0" right="94.0" top="12.0" />
+               </padding>
+               <VBox.margin>
+                  <Insets top="20.0" />
+               </VBox.margin>
+            </Button>
          </children>
       </VBox>
    </left>