优化UI样式：调整头部背景色和按钮焦点样式

src/renderer/App.css

@@ -5,7 +5,7 @@
 }
 
 .app-header {
-  background: #2c3e50;
+  background: #3498db;
   color: white;
   padding: 1rem 2rem;
   display: flex;
@@ -56,6 +56,10 @@
   background: #229954;
 }
 
+.add-btn:focus {
+  outline: none;
+}
+
 .app-main {
   flex: 1;
   padding: 2rem;