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.',