fix(ci): perf job — point Lighthouse at /fr/ and /en/, drop spa fallback (#92)

## Summary

The previous PR (#91) enabled `--localize` on the production build, so the output layout became `dist/apps/portal-shell/browser/{en,fr}/` with **no top-level `index.html`**. The `perf` CI job broke in two places downstream:

1. **`nx run portal-shell:serve-static`** had `spa: true`. The `@nx/web:file-server` executor reads that as "copy `<staticFilePath>/index.html` to `404.html` for SPA fallback". The source file no longer exists, so the executor crashed with `ENOENT … copyfile … index.html` before opening the port. lhci then failed its healthcheck and exited 1.
2. **`lighthouserc.js`** was hitting `http://localhost:4200/`, which now lands on `http-server`'s directory listing (no index.html at that path). Even if the server had started, the audit would have measured the wrong page.

## What changes

- **Drop `spa: true`** from the `serve-static` target in [`apps/portal-shell/project.json`](apps/portal-shell/project.json). Deep-link fallback in production is the reverse proxy's job (it routes `/{en,fr}/anything` to the matching `index.html`); `nx serve-static` is only used here for the perf gate and for local prod-build inspection of entry points. For deep-link testing in dev, `nx serve` is the right tool.
- **Update [`lighthouserc.js`](lighthouserc.js)** `url` list to `['http://localhost:4200/fr/', 'http://localhost:4200/en/']`, matching the directive in ADR-0019 that both locales clear the same performance bar.

## Verification

Local repro (against the merged plumbing PR's build):

```
$ pnpm exec nx run portal-shell:serve-static
$ curl -s -o /dev/null -w '%{http_code}\n' http://localhost:4200/en/   # 200
$ curl -s -o /dev/null -w '%{http_code}\n' http://localhost:4200/fr/   # 200
```

Served files have the right metadata per locale:

```
/tmp/probe-en.html: lang="en"   <base href="/en/">
/tmp/probe-fr.html: lang="fr"   <base href="/fr/">
```

## Side-effect to call out

- `/en/deep/route` and `/fr/deep/route` now return 404 from `nx serve-static`. That's by design — Lighthouse only audits the root locale URLs, and the reverse proxy owns deep-link routing in production.
- `http://localhost:4200/` returns http-server's directory listing under the new layout. Lighthouse doesn't hit it, so the perf gate is unaffected. We could disable the listing if it becomes a footgun.

## Test plan

- [x] `pnpm exec nx run-many -t lint test build --projects=portal-shell` — green.
- [x] Local `nx serve-static` + curl against `/en/` and `/fr/` returns the expected per-locale `index.html`.
- [ ] CI: `pnpm ci:perf` runs through `serve-static` start → Lighthouse autorun (×3 per locale, ×2 locales = 6 audits) → assertions hold ≥ 90 on Performance for both.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #92

apps/portal-shell/project.json

@@ -112,8 +112,7 @@
       "options": {
         "buildTarget": "portal-shell:build",
         "port": 4200,
-        "staticFilePath": "dist/apps/portal-shell/browser",
-        "spa": true
+        "staticFilePath": "dist/apps/portal-shell/browser"
       }
     }
   }