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/tooling/viteBuild.test.mjs

@@ -65,6 +65,10 @@ test('explicit build inputs preserve browser-only defines, plugin order and loop
   assert.ok(!csp.includes("script-src 'self' 'unsafe-inline'"));
   assert.deepEqual(config.preview.headers, config.server.headers);
   assert.deepEqual(config.define, {
+    __VUE_I18N_FULL_INSTALL__: 'false',
+    __VUE_I18N_LEGACY_API__: 'false',
+    __INTLIFY_PROD_DEVTOOLS__: 'false',
+    __INTLIFY_DROP_MESSAGE_COMPILER__: 'true',
     'import.meta.env.GOOGLE_MAPS_API_KEY': '"browser-fixture"',
     'import.meta.env.CESIUM_ION_TOKEN': '"ion-fixture"',
     'import.meta.env.MAPILLARY_CLIENT_TOKEN': '""',