feat(infra): local serve-static profile — Caddy reverse proxy for the prod build

Add a Caddy reverse proxy behind a new `--profile serve-static` so a
contributor can exercise the production build with the per-locale
routing that the on-prem reverse proxy will use (ADR-0019). Closes
the gap PR #96 surfaced: the locale switcher / route fusion / cookie
plumbing all need a prod-faithful local setup, and `nx serve-static`
falls short (no SPA fallback per locale, no smart `/` redirect).

What lands:

- `infra/local/Caddyfile` — three-block route:
    * `GET /` → smart redirect to `/{locale}/` based on
      `Accept-Language` (FR fallback, since the APF audience is
      francophone). Cookie support waits on the BFF route in
      ADR-0019.
    * `/fr/*` → serves `dist/.../browser/fr/` with SPA fallback to
      `index.html` for deep links.
    * `/en/*` → mirror.
    * Catch-all → bounces to `/fr/` so a typo can't land on the
      Caddy directory-listing footgun (same family as the perf-gate
      fix in #92).
- `infra/local/dev.compose.yml` — new `serve-static` service on the
  `serve-static` profile, bind-mounting the Caddyfile and the
  `dist/apps/portal-shell/browser/` folder (read-only). Default
  port 4200, override via `SERVE_STATIC_PORT` in `.env`.
- `infra/local/.env.example` — adds `SERVE_STATIC_PORT=4200`.
- `infra/local/dev.sh` — registers `serve-static` in
  `ALL_PROFILES` so `dev.sh down|status|logs` catches the new
  container, and `dev.sh up serve-static` works.
- `infra/README.md` — adds the new file row, the workflow in the
  "First-time setup" snippet, the cheat-sheet row, and the service
  endpoint row with the prerequisite `nx build … -c=production`.

Workflow once merged:

```
pnpm exec nx build portal-shell --configuration=production
./infra/local/dev.sh up serve-static
open http://localhost:4200/   # → /fr/ or /en/ per Accept-Language
```

Verified locally:

- GET / with Accept-Language=fr → 302 /fr/
- GET / with Accept-Language=en → 302 /en/
- GET /unknown → 302 /fr/
- GET /fr/deep/route → 200 (SPA fallback to fr/index.html)
- GET /fr/favicons/favicon.svg → 200 (per-locale asset served)
- /fr/index.html has `lang="fr"` and `<base href="/fr/">`
- /en/index.html has `lang="en"` and `<base href="/en/">`

This is local convenience only — no TLS, no auth, binds to
localhost. The on-prem reverse proxy gets its own ADR (phase 3b).

infra/local/dev.sh

@@ -16,7 +16,7 @@ COMPOSE_FILE="${SCRIPT_DIR}/dev.compose.yml"
 
 # Profiles defined in dev.compose.yml. Keep in sync if a new profile
 # is added.
-ALL_PROFILES=(dbtools observability)
+ALL_PROFILES=(dbtools observability serve-static)
 
 # Build "--profile p1 --profile p2 …" as separate arguments.
 build_all_profile_flags() {
@@ -59,6 +59,7 @@ Commands:
                           all            core + all profiles
                           dbtools        core + pgweb
                           observability  core + jaeger
+                          serve-static   core + caddy (production-build reverse proxy)
                           Multiple targets allowed (e.g. `up dbtools observability`).
 
   down [-v]             Tear the stack down. Always runs with every