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.

scripts/i18n.mjs

@@ -156,7 +156,7 @@ const SKIP_TAGS = new Set(['script', 'style', 'svg', 'code', 'pre', 'title']);
 /**
  * User-visible markup text without data-i18n*. Skipped, with everything
  * inside: translate="no" (text that stays as is, as in HTML),
- * data-i18n-runtime (code sets this text through t()), and icons
+ * data-i18n-runtime (page markup whose text code sets through t()), and icons
  * (aria-hidden, Material Symbols).
  */
 export function unmarkedMarkup(file, html) {
@@ -404,8 +404,13 @@ export function unmarkedScript(file, original) {
  */
 export function findUnmarked() {
   const found = [];
-  for (const file of markupFiles())
+  for (const file of markupFiles()) {
     found.push(...unmarkedMarkup(file, readMarkup(file)));
+  }
+  for (const file of trackedFiles('src/**/*.vue'))
+    found.push(
+      ...markedTemplate(file, readFileSync(path.join(ROOT, file), 'utf8')),
+    );
   for (const file of sourceFiles().filter(
     (name) => !NOT_UI.some((pattern) => pattern.test(name)),
   ))
@@ -421,7 +426,38 @@ export function findUnmarked() {
   });
 }
 
-/** Static markup opted in with data-i18n*: the page and its views. */
+/**
+ * data-i18n* in a Vue template: views translate with t() as they render, so
+ * their text follows the locale; the attributes are for static page markup.
+ * Reads the whole single-file component, so lines are the file's own.
+ */
+export function markedTemplate(file, source) {
+  const start = source.indexOf('<template>');
+  if (start === -1) return [];
+  const end = source.lastIndexOf('</template>');
+  // Comments and attribute values are blanked, keeping every offset.
+  const blank = (text) => text.replace(/[^\n]/g, ' ');
+  const template = source
+    .slice(0, end === -1 ? undefined : end)
+    .replace(/<!--[\s\S]*?-->/g, blank);
+  const found = [];
+  const TAG = /<[a-zA-Z][\w-]*((?:"[^"]*"|'[^']*'|[^>"'])*)>/g;
+  TAG.lastIndex = start + '<template>'.length;
+  for (let tag; (tag = TAG.exec(template));) {
+    const attrs = tag[1].replace(/"[^"]*"|'[^']*'/g, blank);
+    const offset = tag.index + tag[0].length - 1 - tag[1].length;
+    for (const match of attrs.matchAll(
+      /(?<=\s)(?::|v-bind:)?(data-i18n[\w-]*)(?=[\s=/]|$)/g,
+    ))
+      found.push({
+        where: `${file}:${lineAt(source, offset + match.index)}`,
+        text: `${match[1]}: use t() in a Vue template`,
+      });
+  }
+  return found;
+}
+
+/** Markup checked for unmarked text: the page and its views. */
 function markupFiles() {
   return trackedFiles('index.html', 'src/ui/views/*.vue');
 }