i18n: translate the application views with t() as they render

Every data-i18n, data-i18n-title, data-i18n-aria-label and
data-i18n-placeholder marker in the Vue templates becomes a t() call with
the key the boot pass used to look up, contexts included. The three labels
the shell rewrote at boot (Visual Presets, Location and the standby
description) render through t() too, and that rewrite is gone. With the
locale in vue-i18n, view text now follows a locale change, and markup
under v-if or v-for no longer comes back in English when it re-renders.

Rendered text and attributes are unchanged in every locale. The boot pass
still translates static page markup such as the loading screen and a host's
own markup, and npm run i18n rejects data-i18n* in a Vue template.

src/standalone/application.js

@@ -29,12 +29,13 @@ export function createStandaloneApplication({
   constructed = true;
   // Locale is resolved before any UI initialization so <html lang>/<html dir>
   // and every catalog lookup agree from the first painted frame. Module
-  // scripts execute after parsing, so the DOM is ready here and every
-  // data-i18n* element honors the resolved locale without a reload.
+  // scripts execute after parsing, so the DOM is ready here: the views
+  // translate as they render, and the page's static data-i18n* markup (the
+  // loading screen) is translated once.
   const activeLocale = resolveLocale();
   setLocale(activeLocale);
   applyDocumentLanguage(document, activeLocale);
-  // Views render the page's markup; translation and wiring read it.
+  // Views render the page's markup; wiring reads it.
   mountApplicationViews(document);
   applyDocumentTranslations(document);
   const loadingScreen = document.getElementById('loading-screen');