style: 更新全局样式和主题变量

- 添加新的 CSS 变量
- 优化暗色主题样式

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/style.css

@@ -120,7 +120,15 @@ html.dark .el-select .el-select__wrapper .el-select__selected-item {
 }
 
 html.dark .el-select .el-input__inner::placeholder {
-  color: #6b7280;
+  color: #6b7280 !important;
+}
+
+html.dark .el-select .el-select__placeholder {
+  color: #6b7280 !important;
+}
+
+html.dark .el-input__inner::placeholder {
+  color: #6b7280 !important;
 }
 
 /* 下拉箭头 */