feat(spa): proxy /api in dev-server, relative bffApiBaseUrl

Make the SPAs reach the BFF same-origin via the Angular dev-server's
proxy, so the dockerised dev mode (ADR-0030) works when the SPA is
accessed from a remote browser (e.g. http://<vm-ip>:4200/) — and CORS
is bypassed in dev altogether.

- New proxy.conf.js per SPA: /api -> ${BFF_TARGET:-http://localhost:3000}
  (JS form so the env var can swap the target at startup without a
  rebuild).
- project.json serve.options.proxyConfig wired in for both apps.
- environment.ts (shell + admin) bffApiBaseUrl: 'http://localhost:3000/api'
  -> '/api'. Production siblings can still set an absolute origin if
  the SPA and BFF live on different hosts; tracing.ts resolves either
  form against window.location.origin.
- tracing.ts: new URL(env, window.location.origin) — relative bases no
  longer throw, absolute bases keep their own origin.
- dev.compose.yml: BFF_TARGET=http://portal-bff:3000 on portal-shell
  and portal-admin so the proxy hits the BFF container by Compose DNS.
  Native nx serve leaves it unset and falls back to localhost.

OTel exporter URL and cross-SPA links remain absolute — same remote-
browser issue, but not blocking the 'Backend unreachable' path this
PR targets. Out of scope here, follow-up if needed.

Stacked on top of feat/dockerised-dev-mode.

apps/portal-shell/project.json

@@ -83,6 +83,9 @@
     "serve": {
       "continuous": true,
       "executor": "@angular/build:dev-server",
+      "options": {
+        "proxyConfig": "apps/portal-shell/proxy.conf.js"
+      },
       "configurations": {
         "production": {
           "buildTarget": "portal-shell:build:production"