style(nav): 对齐四 Tab 底栏布局规格

test/theme/app_dimens_test.dart

@@ -19,11 +19,12 @@ void main() {
     expect(AppDimens.sectionTitle, 16);
     expect(AppDimens.pageTitle, 20);
     expect(AppDimens.navLabel, 11);
-    expect(AppDimens.navIcon, 24);
+    expect(AppDimens.navLabelGap, 4);
+    expect(AppDimens.navIcon, 26);
     expect(AppDimens.buttonHeight, 48);
     expect(AppDimens.primaryButtonHeight, 52);
     expect(AppDimens.tapTarget, 44);
-    expect(AppDimens.navHeight, 68);
+    expect(AppDimens.navHeight, 56);
     expect(AppDimens.progressHeight, 3);
   });
 }