i18n: keep the page's locale in a vue-i18n instance

The application views install one vue-i18n instance that holds the active
locale and the catalogs. src/i18n/index.js stays free of Vue: bindLocale()
keeps its locale in the instance's ref, so t(), tn() and the Intl formatters
read it from there and a view that translates while rendering renders again
when the locale changes. A tag written straight to the ref is normalized.
Message formatting, catalogs and call sites are unchanged, and switching
language still reloads the page.

The instance outlives the views: unmounting one no longer disposes it. The
browser build defines vue-i18n's feature flags and drops its message
compiler, which Core does not use.

src/ui/i18n.js

@@ -0,0 +1,46 @@
+import { createI18n } from 'vue-i18n';
+import { bindLocale, getCatalog, getLocale } from '../i18n/index.js';
+import { DEFAULT_LOCALE, LOCALES } from '../i18n/locale.js';
+
+let pageI18n = null;
+
+// Catalogs are flat and keyed by English text, which holds dots.
+const resolveFlat = (messages, key) =>
+  Object.hasOwn(messages, key) ? messages[key] : null;
+
+/**
+ * The page's vue-i18n instance, installed in every application view. It
+ * holds the active locale and the catalogs; Core's t(), tn() and the Intl
+ * formatters read the locale from it, so a view that calls them while
+ * rendering re-renders when the locale changes. Views translate through
+ * t()/tn() rather than vue-i18n's own t: English is the key, `context` picks
+ * an entry, and plural forms follow Intl.PluralRules (docs/I18N.md).
+ * @returns {import('vue-i18n').I18n}
+ */
+export function applicationI18n() {
+  if (pageI18n) return pageI18n;
+  const messages = {};
+  for (const locale of LOCALES)
+    if (locale !== DEFAULT_LOCALE) messages[locale] = getCatalog(locale);
+  pageI18n = createI18n({
+    legacy: false,
+    globalInjection: false,
+    locale: getLocale(),
+    fallbackLocale: DEFAULT_LOCALE,
+    messages,
+    messageResolver: resolveFlat,
+    missingWarn: false,
+    fallbackWarn: false,
+  });
+  // vue-i18n disposes its instance when an app it is installed in unmounts;
+  // the page's outlives every view, so an unmounted panel keeps it.
+  const install = pageI18n.install;
+  pageI18n.install = (app, ...options) => {
+    const unmount = app.unmount;
+    const installed = install.call(pageI18n, app, ...options);
+    app.unmount = unmount;
+    return installed;
+  };
+  bindLocale(pageI18n.global.locale);
+  return pageI18n;
+}