feat(i18n): locale core + English catalogs + parity gates

Add the i18n foundation: src/i18n/locale.js (locale resolution with a
configurable GEV_DEFAULT_LOCALE / GEV_SECONDARY_LOCALE pair, guarded
preference storage, one-shot ?lang= override, <html lang>/<html dir>
metadata) and src/i18n/index.js (catalog registry, t(), Intl formatters,
applyDocumentTranslations, persistLocaleAndReload).

English is the only shipped locale in this PR: the four en catalogs
(shell/cockpit/layers/setup) are the reference and the always-shipped
fallback, the built-in pair degenerates to en-only, and English output
is byte-identical to the strings these replace.

Parity gates land with the core (catalog key/placeholder/plural-shape
parity, markup coverage of every data-i18n* attribute, repair-pass
anchors pinning en byte-identity); additional locales ship as stacked
follow-up PRs through the recipe in docs/TRANSLATORS.md.

Rebased onto current main: the client defines live in build/vite.js /
server/standalone/vite.config.js as before.

server/standalone/vite.config.js

@@ -16,6 +16,8 @@ export default defineConfig(({ command, mode }) => {
     plugins: [...localProviderPlugins(), apiNotFoundPlugin()],
     googleApiKey: process.env.GOOGLE_MAPS_API_KEY,
     cesiumToken: process.env.CESIUM_ION_TOKEN,
+    defaultLocale: process.env.GEV_DEFAULT_LOCALE ?? '',
+    secondaryLocale: process.env.GEV_SECONDARY_LOCALE ?? '',
     host: process.env.HOST,
     port: process.env.PORT,
     command,