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/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()