fix(frontend): localize stray hardcoded UI strings

Move the last few user-visible hardcoded strings into vue-i18n:

- AppHeader: "Toggle Menu" / "User Menu" aria-labels (screen readers
  read these in English regardless of locale)
- CustomPageView: "Page not found" error body
- PaymentProviderDialog: display-name example placeholder hardcoded in
  Chinese for all locales

Adds common.toggleMenu / common.userMenu / common.pageNotFound and
admin.settings.payment.customMethodDisplayNamePlaceholder to both zh
and en locales, and registers the new placeholder key in the
PaymentProviderDialog spec's i18n mock.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/layout/AppHeader.vue

@@ -6,7 +6,7 @@
         <button
           @click="toggleMobileSidebar"
           class="btn-ghost btn-icon lg:hidden"
-          aria-label="Toggle Menu"
+          :aria-label="t('common.toggleMenu')"
         >
           <Icon name="menu" size="md" />
         </button>
@@ -96,7 +96,7 @@
           <button
             @click="toggleDropdown"
             class="flex items-center gap-2 rounded-xl p-1.5 transition-colors hover:bg-gray-100 dark:hover:bg-dark-800"
-            aria-label="User Menu"
+            :aria-label="t('common.userMenu')"
           >
             <div class="flex h-8 w-8 items-center justify-center overflow-hidden rounded-xl bg-gradient-to-br from-primary-500 to-primary-600 text-sm font-medium text-white shadow-sm">
               <img

frontend/src/components/payment/PaymentProviderDialog.vue

@@ -100,7 +100,7 @@
             </div>
             <div>
               <label class="text-xs text-gray-500 dark:text-gray-400">{{ t('admin.settings.payment.customMethodDisplayName') }}</label>
-              <input v-model="method.displayName" type="text" class="input mt-0.5" placeholder="信用卡" />
+              <input v-model="method.displayName" type="text" class="input mt-0.5" :placeholder="t('admin.settings.payment.customMethodDisplayNamePlaceholder')" />
             </div>
             <button
               type="button"

frontend/src/components/payment/__tests__/PaymentProviderDialog.spec.ts

@@ -13,6 +13,7 @@ const messages: Record<string, string> = {
   'admin.settings.payment.customMethodType': 'Payment type',
   'admin.settings.payment.customMethodUpstreamType': 'Upstream type',
   'admin.settings.payment.customMethodDisplayName': 'Display name',
+  'admin.settings.payment.customMethodDisplayNamePlaceholder': '信用卡',
   'admin.settings.payment.paymentGuideTrigger': 'View payment guide',
   'admin.settings.payment.alipayGuideSummary': 'Desktop prefers QR precreate and falls back to cashier; mobile prefers WAP checkout.',
   'admin.settings.payment.wxpayGuideSummary': 'Desktop prefers Native QR; mobile routes to JSAPI or H5 based on browser context.',

frontend/src/i18n/locales/en/admin/settings.ts

@@ -642,6 +642,7 @@ export default {
         customMethodType: 'Payment type',
         customMethodUpstreamType: 'Upstream type',
         customMethodDisplayName: 'Display name',
+        customMethodDisplayNamePlaceholder: 'e.g. Credit card',
         stripeWebhookHint: 'Configure the following URL as a Webhook endpoint in Stripe Dashboard:',
         stripeWebhookApiVersionHint: 'Set this Webhook endpoint API version to match the integrated Stripe SDK. Recommended: {version}. A mismatch can cause webhook parsing errors.',
         airwallexWebhookHint: 'Configure the following URL as a Webhook endpoint in Airwallex. Select at least Payment Intent -> Succeeded (payment_intent.succeeded), preferably also Payment Intent -> Cancelled (payment_intent.cancelled). Use the account default or latest stable API version.',

frontend/src/i18n/locales/en/common.ts

@@ -44,6 +44,9 @@ export default {
     inactive: 'Inactive',
     more: 'More',
     close: 'Close',
+    toggleMenu: 'Toggle menu',
+    userMenu: 'User menu',
+    pageNotFound: 'Page not found',
     enabled: 'Enabled',
     disabled: 'Disabled',
 	    total: 'Total',

frontend/src/i18n/locales/zh/admin/settings.ts

@@ -637,6 +637,7 @@ export default {
         customMethodType: '支付方式',
         customMethodUpstreamType: '上游 type',
         customMethodDisplayName: '显示名称',
+        customMethodDisplayNamePlaceholder: '如：信用卡',
         stripeWebhookHint: '请在 Stripe Dashboard 中将以下地址配置为 Webhook 端点：',
         stripeWebhookApiVersionHint: 'Webhook 端点的 API 版本请与当前集成的 Stripe SDK 对齐，建议选择 {version}；版本不一致可能导致回调事件解析失败。',
         airwallexWebhookHint: '请在 Airwallex 后台将以下地址配置为 Webhook 端点；事件至少选择 Payment Intent -> Succeeded（payment_intent.succeeded），建议同时选择 Payment Intent -> Cancelled（payment_intent.cancelled）；API version 选择账户默认或最新稳定版本。',

frontend/src/i18n/locales/zh/common.ts

@@ -44,6 +44,9 @@ export default {
     inactive: '禁用',
     more: '更多',
     close: '关闭',
+    toggleMenu: '切换菜单',
+    userMenu: '用户菜单',
+    pageNotFound: '页面不存在',
     enabled: '已启用',
     disabled: '已禁用',
 	    total: '总计',

frontend/src/views/user/CustomPageView.vue

@@ -230,7 +230,7 @@ async function fetchAndRenderMarkdown(slug: string) {
       headers: authStore.token ? { Authorization: `Bearer ${authStore.token}` } : {},
     })
     if (!resp.ok) {
-      renderedHtml.value = '<p class="text-red-500">Page not found</p>'
+      renderedHtml.value = `<p class="text-red-500">${t('common.pageNotFound')}</p>`
       return
     }
     let raw = await resp.text()