feat(i18n): per-bundle LOCALE_ID + language switching under docker compose

Fix the hardcoded LOCALE_ID: 'nl' — provide it from $localize.locale (the build-time
locale, 'nl'/'en', undefined→'nl' in dev) and register both nl+en locale data, so the en
bundle formats dates/numbers correctly. Make `docker compose up` serve the LOCALIZED build:
the web service now runs `ng build --localize` then serve-i18n.mjs, which gained a PORT env
+ an /api reverse-proxy (API_PROXY_TARGET → the api container) so both /nl/ and /en/ are
served with the language switcher working end-to-end. Drop the now-unused proxy.conf.docker.json
(serve-i18n proxies /api itself); update ARCHITECTURE. `npm start` stays the nl-only HMR loop.

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

docker-compose.yml

@@ -1,5 +1,8 @@
 # ponytail: dev-server images (not multi-stage prod builds) — this is a demo.
-# `docker compose up` → app at http://localhost:4200, Swagger at http://localhost:5000/swagger
+# `docker compose up` → app at http://localhost:4200 (LOCALIZED: /nl/ + /en/, the header
+# language switcher works), Swagger at http://localhost:5000/swagger. The web container does a
+# one-time `ng build --localize` then serves both locale bundles statically (with /api proxied);
+# for a fast HMR loop use `npm start` locally instead (nl-only at /).
 services:
   api:
     image: mcr.microsoft.com/dotnet/sdk:10.0
@@ -30,12 +33,14 @@ services:
     # msgpackr-extract) ship prebuilt glibc binaries, so no build toolchain is needed.
     image: node:24-slim
     working_dir: /app
-    # Uses the committed generated client (no codegen at startup); proxies /api → api container.
-    # ponytail: `--no-fund --loglevel=error` silences npm 11's startup noise (deprecation +
-    # allow-scripts + funding) so the demo boots clean; ng serve output is unaffected. The
-    # deprecation lines are Angular 22's webpack-builder (@angular-devkit/build-angular) — a
-    # known upstream deprecation; migrating to @angular/build is a separate, larger effort.
-    command: sh -c "npm ci --no-fund --loglevel=error && npx ng serve --host 0.0.0.0 --proxy-config proxy.conf.docker.json"
+    # Uses the committed generated client (no codegen at startup). Builds BOTH locale bundles
+    # once (`ng build --localize`) then serves them statically via scripts/serve-i18n.mjs
+    # (per-locale SPA fallback + /api reverse-proxy → the api container), so the language
+    # switcher actually switches. ponytail: `--no-fund --loglevel=error` silences npm 11 noise.
+    command: sh -c "npm ci --no-fund --loglevel=error && npx ng build --localize && node scripts/serve-i18n.mjs"
+    environment:
+      - PORT=4200
+      - API_PROXY_TARGET=http://api:5000
     volumes:
       - ./:/app:z
       - web-modules:/app/node_modules