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.

CONTRIBUTING.md

@@ -126,7 +126,7 @@ The highest-leverage places to jump in:
 - ES modules, **2-space indent, single quotes, semicolons.**
 - JSDoc on exported/public functions.
 - Match the surrounding code — comment density, naming, and idiom.
-- User-visible text goes through `t()` or `tn()`, with `msg()` marking English that is stored and translated later. Static page markup that predates the views keeps its `data-i18n*` attributes; components use `t()`. `npm run i18n` must pass. See [docs/I18N.md](docs/I18N.md).
+- User-visible text goes through `t()` or `tn()`, with `msg()` marking English that is stored and translated later. Components call `t()` in their templates, so their text follows the locale; `data-i18n*` attributes are only for static page markup such as the loading screen, and `npm run i18n` rejects them in a template. `npm run i18n` must pass. See [docs/I18N.md](docs/I18N.md).
 - Prefer small, reviewable commits. Conventional-commit-style prefixes (`feat:`, `fix:`, `perf:`, `docs:`) are appreciated but not required.
 
 ## Building UI