feat: Introduce application version display in the frontend footer, showing both frontend and backend versions with integrated build arguments.

backend/src/main/java/com/todo/backend/controller/VersionController.java

@@ -0,0 +1,25 @@
+package com.todo.backend.controller;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@RestController
+@RequestMapping("/api/version")
+public class VersionController {
+
+    @Value("${APP_VERSION:dev}")
+    private String appVersion;
+
+    @GetMapping
+    public ResponseEntity<Map<String, String>> getVersion() {
+        Map<String, String> response = new HashMap<>();
+        response.put("version", appVersion);
+        return ResponseEntity.ok(response);
+    }
+}