fix(payment): audit fixes for alipay/wxpay/stripe payment providers

Backend:
- Extract YuanToFen/FenToYuan to payment/amount.go using shopspring/decimal
- Require alipay publicKey in config validation
- Fix wxpay webhook response to return JSON per V3 spec
- Remove wxpay certSerial fallback to publicKeyId
- Define magic strings as named constants in wxpay/alipay providers
- Add slog warning for wxpay H5→Native payment downgrade
- Make EncryptionKey validation return error on invalid (non-empty) key
- Make decryptConfig propagate errors instead of returning nil
- Add idempotency check in doBalance to prevent stuck FAILED retries

Frontend:
- Fix dashboard currency symbol from $ to ¥
- Fix AdminPaymentPlansView any type to proper SubscriptionPlan type
- Make quick amount buttons follow selected payment method limits
- Center help image with larger height and text below

frontend/src/views/admin/orders/AdminPaymentDashboardView.vue

@@ -42,7 +42,7 @@
                   <span class="text-sm text-gray-700 dark:text-gray-300">{{ t('payment.methods.' + method.type, method.type) }}</span>
                 </div>
                 <div class="text-right">
-                  <span class="text-sm font-medium text-gray-900 dark:text-white">${{ method.amount.toFixed(2) }}</span>
+                  <span class="text-sm font-medium text-gray-900 dark:text-white">&yen;{{ method.amount.toFixed(2) }}</span>
                   <span class="ml-2 text-xs text-gray-500 dark:text-gray-400">({{ method.count }})</span>
                 </div>
               </div>
@@ -57,7 +57,7 @@
                   <span :class="['flex h-6 w-6 items-center justify-center rounded-full text-xs font-bold', rankClass(idx)]">{{ idx + 1 }}</span>
                   <span class="text-sm text-gray-700 dark:text-gray-300">{{ user.email }}</span>
                 </div>
-                <span class="text-sm font-medium text-gray-900 dark:text-white">${{ user.amount.toFixed(2) }}</span>
+                <span class="text-sm font-medium text-gray-900 dark:text-white">&yen;{{ user.amount.toFixed(2) }}</span>
               </div>
             </div>
           </div>