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.

package.json

@@ -68,7 +68,8 @@
     "pbf": "^5.1.2",
     "pinia": "^2.3.1",
     "satellite.js": "^6.0.2",
-    "vue": "^3.5.44"
+    "vue": "^3.5.44",
+    "vue-i18n": "^11.4.15"
   },
   "devDependencies": {
     "@happy-dom/global-registrator": "^20.14.6",