feat(i18n): nl at root URLs + keep devtools in the docker demo

Serve the source locale (nl) at / instead of /nl/: angular.json sourceLocale is now
{ code: 'nl', subPath: '' } → nl output at browser/ root (base href /), en stays /en/.
Rework localeLinks (nl → bare path, en → /en/…) + spec, and serve-i18n.mjs (nl assets at
root, en under /en/, / serves the nl index). And build the docker demo (+ serve:i18n) with
`--configuration development --localize` so isDevMode() stays true and the dev `⚙ state`
panel + role/scenario switchers render in the localized compose demo (they were correctly
gated off in the previous production build). Verified: nl base href /, en /en/, ngDevMode present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

angular.json

@@ -18,7 +18,7 @@
       "sourceRoot": "src",
       "prefix": "app",
       "i18n": {
-        "sourceLocale": "nl",
+        "sourceLocale": { "code": "nl", "subPath": "" },
         "locales": {
           "en": {
             "translation": "src/locale/messages.en.xlf"