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.

changelog.d/0000-vue-and-pinia.md

@@ -1,3 +1,9 @@
+- The application views translate their text with `t()` as they render,
+  so it follows the active locale; they no longer carry `data-i18n*`
+  attributes, and `npm run i18n` rejects those in a Vue template. The boot
+  pass (`applyDocumentTranslations`) still translates static page markup
+  such as the loading screen and a host's own markup (samehkhamis, #0000).
+
 - The page's locale lives in a vue-i18n instance installed in every
   application view, and `t()`, `tn()` and the Intl formatters read it from
   there, so a view that translates while rendering renders again when the