fix: 修复暗色主题下下拉菜单白色背景闪烁问题

- 添加 dark-select-dropdown 自定义类
- 修复鼠标移出后下拉菜单出现白色背景的问题
- 强制覆盖所有可能的白色背景样式

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

web/src/views/Database.vue

@@ -807,7 +807,7 @@ const filteredDatabases = () => {
 
             <div>
               <label class="block text-sm font-medium text-gray-300 mb-2">Type</label>
-              <el-select v-model="editForm.db_type" placeholder="Select" class="w-full" size="large">
+              <el-select v-model="editForm.db_type" placeholder="Select" class="w-full" size="large" popper-class="dark-select-dropdown">
                 <el-option v-for="type in dbTypes" :key="type" :label="type" :value="type" />
               </el-select>
             </div>
@@ -924,7 +924,7 @@ const filteredDatabases = () => {
 
             <div>
               <label class="block text-sm font-medium text-gray-300 mb-2">Type</label>
-              <el-select v-model="newDbForm.db_type" placeholder="Select" class="w-full" size="large">
+              <el-select v-model="newDbForm.db_type" placeholder="Select" class="w-full" size="large" popper-class="dark-select-dropdown">
                 <el-option v-for="type in dbTypes" :key="type" :label="type" :value="type" />
               </el-select>
             </div>