feat(payment): add recharge fee rate setting and fix provider card UI

- Add recharge_fee_rate system setting (percentage fee on top of recharge amount)
- Full backend chain: config constant, PaymentConfig struct, update validation,
  read/write persistence, DTO, handler GET/PUT responses
- Frontend: settings input with preview, i18n (zh/en), API types
- Fix provider card toggle layout: labels above switches to save width
- Fix Chinese translation: "EasyPay" → "易支付" in provider description

frontend/src/api/admin/settings.ts

@@ -126,6 +126,7 @@ export interface SystemSettings {
   payment_enabled_types: string[]
   payment_balance_disabled: boolean
   payment_balance_recharge_multiplier: number
+  payment_recharge_fee_rate: number
   payment_load_balance_strategy: string
   payment_product_name_prefix: string
   payment_product_name_suffix: string
@@ -233,6 +234,7 @@ export interface UpdateSettingsRequest {
   payment_enabled_types?: string[]
   payment_balance_disabled?: boolean
   payment_balance_recharge_multiplier?: number
+  payment_recharge_fee_rate?: number
   payment_load_balance_strategy?: string
   payment_product_name_prefix?: string
   payment_product_name_suffix?: string

frontend/src/components/payment/ToggleSwitch.vue

@@ -1,6 +1,6 @@
 <template>
-  <label class="flex items-center gap-1.5 cursor-pointer">
-    <span class="text-xs text-gray-500 dark:text-gray-400">{{ label }}</span>
+  <label class="flex flex-col items-center gap-0.5 cursor-pointer">
+    <span class="text-xs text-gray-500 dark:text-gray-400 whitespace-nowrap">{{ label }}</span>
     <button
       type="button"
       role="switch"

frontend/src/i18n/locales/en.ts

@@ -4569,6 +4569,9 @@ export default {
         balanceRechargeMultiplier: 'Balance Recharge Multiplier',
         balanceRechargeMultiplierHint: 'How many USD balance the user receives for each 1 CNY paid',
         balanceRechargePreview: 'Preview: 1 CNY = {usd} USD',
+        rechargeFeeRate: 'Recharge Fee Rate',
+        rechargeFeeRateHint: 'Percentage of service fee charged on top of recharge amount, 0 means no fee',
+        rechargeFeePreview: 'Preview: Recharge 100, fee {fee}',
         orderTimeout: 'Order Timeout',
         orderTimeoutHint: 'In minutes, minimum 1',
         maxPendingOrders: 'Max Pending Orders',

frontend/src/i18n/locales/zh.ts

@@ -4726,13 +4726,16 @@ export default {
         enabledHint: '启用或禁用支付系统',
         enabledPaymentTypes: '启用的服务商',
         enabledPaymentTypesHint: '禁用服务商将同时禁用对应的实例。',
-        findProvider: '正在寻找合适的 EasyPay 服务商？',
+        findProvider: '正在寻找合适的易支付服务商？',
         minAmount: '最低金额',
         maxAmount: '最高金额',
         dailyLimit: '每日限额',
         balanceRechargeMultiplier: '余额充值倍率',
         balanceRechargeMultiplierHint: '用户每支付 1 CNY 可获得多少 USD 余额',
         balanceRechargePreview: '预览：1 CNY = {usd} USD',
+        rechargeFeeRate: '充值手续费率',
+        rechargeFeeRateHint: '用户充值时额外收取的手续费百分比，0 表示不收取手续费',
+        rechargeFeePreview: '预览：充值 100 元，手续费 {fee} 元',
         orderTimeout: '订单超时时间',
         orderTimeoutHint: '单位：分钟，至少 1 分钟',
         maxPendingOrders: '最大待支付订单数',

frontend/src/views/admin/SettingsView.vue

@@ -2381,6 +2381,12 @@
                   <p class="mt-0.5 text-xs text-gray-400">{{ t('admin.settings.payment.balanceRechargeMultiplierHint') }}</p>
                   <p class="mt-1 text-xs font-medium text-primary-600 dark:text-primary-400">{{ t('admin.settings.payment.balanceRechargePreview', { usd: (Number(form.payment_balance_recharge_multiplier) || 1).toFixed(2) }) }}</p>
                 </div>
+                <div>
+                  <label class="input-label">{{ t('admin.settings.payment.rechargeFeeRate') }}</label>
+                  <input :value="form.payment_recharge_fee_rate || ''" @input="form.payment_recharge_fee_rate = parseFloat(($event.target as HTMLInputElement).value) || 0" type="number" step="0.01" min="0" class="input" />
+                  <p class="mt-0.5 text-xs text-gray-400">{{ t('admin.settings.payment.rechargeFeeRateHint') }}</p>
+                  <p v-if="(Number(form.payment_recharge_fee_rate) || 0) > 0" class="mt-1 text-xs font-medium text-primary-600 dark:text-primary-400">{{ t('admin.settings.payment.rechargeFeePreview', { fee: (Number(form.payment_recharge_fee_rate) || 0).toFixed(2) }) }}</p>
+                </div>
                 <div><label class="input-label">{{ t('admin.settings.payment.orderTimeout') }} <span class="text-red-500">*</span></label><input v-model.number="form.payment_order_timeout_minutes" type="number" min="1" class="input" required /><p class="mt-0.5 text-xs text-gray-400">{{ t('admin.settings.payment.orderTimeoutHint') }}</p></div>
               </div>
               <!-- Row 3: Pending orders + load balance + cancel rate limit (all in one row) -->
@@ -2974,7 +2980,7 @@ const form = reactive<SettingsForm>({
   home_content: '',
   backend_mode_enabled: false,
   hide_ccs_import_button: false,
-  payment_enabled: false,  payment_min_amount: 1,  payment_max_amount: 10000,  payment_daily_limit: 50000,  payment_max_pending_orders: 3,  payment_order_timeout_minutes: 30,  payment_balance_disabled: false,  payment_balance_recharge_multiplier: 1,  payment_enabled_types: [],  payment_help_image_url: '',  payment_help_text: '',  payment_product_name_prefix: '',  payment_product_name_suffix: '',  payment_load_balance_strategy: 'round-robin',  payment_cancel_rate_limit_enabled: false,  payment_cancel_rate_limit_max: 10,  payment_cancel_rate_limit_window: 1,  payment_cancel_rate_limit_unit: 'day',  payment_cancel_rate_limit_window_mode: 'rolling',
+  payment_enabled: false,  payment_min_amount: 1,  payment_max_amount: 10000,  payment_daily_limit: 50000,  payment_max_pending_orders: 3,  payment_order_timeout_minutes: 30,  payment_balance_disabled: false,  payment_balance_recharge_multiplier: 1,  payment_recharge_fee_rate: 0,  payment_enabled_types: [],  payment_help_image_url: '',  payment_help_text: '',  payment_product_name_prefix: '',  payment_product_name_suffix: '',  payment_load_balance_strategy: 'round-robin',  payment_cancel_rate_limit_enabled: false,  payment_cancel_rate_limit_max: 10,  payment_cancel_rate_limit_window: 1,  payment_cancel_rate_limit_unit: 'day',  payment_cancel_rate_limit_window_mode: 'rolling',
   table_default_page_size: tablePageSizeDefault,
   table_page_size_options: [10, 20, 50, 100],
   custom_menu_items: [] as Array<{id: string; label: string; icon_svg: string; url: string; visibility: 'user' | 'admin'; sort_order: number}>,
@@ -3634,6 +3640,7 @@ async function saveSettings() {
       payment_order_timeout_minutes: Number(form.payment_order_timeout_minutes) || 0,
       payment_balance_disabled: form.payment_balance_disabled,
       payment_balance_recharge_multiplier: Number(form.payment_balance_recharge_multiplier) || 1,
+      payment_recharge_fee_rate: Number(form.payment_recharge_fee_rate) || 0,
       payment_enabled_types: form.payment_enabled_types,
       payment_load_balance_strategy: form.payment_load_balance_strategy,
       payment_product_name_prefix: form.payment_product_name_prefix,