fix(admin): 修复退款金额精度问题

- 显示完整余额精度，避免四舍五入导致的退款失败
- 添加"全部"按钮，一键填入完整余额
- 移除最小金额限制，支持任意正数金额

frontend/src/i18n/locales/en.ts

@@ -724,6 +724,7 @@ export default {
       withdraw: 'Withdraw',
       depositAmount: 'Deposit Amount',
       withdrawAmount: 'Withdraw Amount',
+      withdrawAll: 'All',
       currentBalance: 'Current Balance',
       depositNotesPlaceholder:
         'e.g., New user registration bonus, promotional credit, compensation, etc.',

frontend/src/i18n/locales/zh.ts

@@ -780,6 +780,7 @@ export default {
       withdraw: '退款',
       depositAmount: '充值金额',
       withdrawAmount: '退款金额',
+      withdrawAll: '全部',
       depositNotesPlaceholder: '例如：新用户注册奖励、活动充值、补偿充值等',
       withdrawNotesPlaceholder: '例如：服务问题退款、错误充值退回、账户注销退款等',
       notesOptional: '备注为可选项，有助于未来查账',