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-admin/proxy.conf.js

@@ -0,0 +1,21 @@
+// Angular dev-server proxy for portal-admin.
+//
+// Mirrors apps/portal-shell/proxy.conf.js — same rationale, same
+// `/api → ${BFF_TARGET:-http://localhost:3000}` rule. The admin app
+// talks to the same BFF (ADR-0020 §"Where does the admin app live"),
+// just at admin-specific paths under `/api/admin/...`; the proxy
+// match on `/api` covers both surfaces.
+//
+// JS form deliberate — only this form can read `process.env` so the
+// Docker / native target swap (BFF_TARGET in dev.compose.yml) works
+// without a rebuild.
+
+const target = process.env['BFF_TARGET'] ?? 'http://localhost:3000';
+
+module.exports = {
+  '/api': {
+    target,
+    secure: false,
+    changeOrigin: true,
+  },
+};