feat(portal-shell): ci gate — fail prod build on missing translations (#98)

## Summary

Set `"i18nMissingTranslation": "error"` on the production build configuration in [`apps/portal-shell/project.json`](apps/portal-shell/project.json). The Angular CLI checks every `<source>` extracted from `i18n` markers and `$localize` calls against the loaded translation file (`messages.fr.xlf`); with the flag on `error`, the build now **fails** when a target is missing instead of silently falling back to the source text in the FR bundle.

Closes the loop on ADR-0019 §"Confirmation" — the final piece of the i18n track that does not depend on the BFF route + cookie work (deferred to the auth-flow chantier).

## Gate mechanics

- `pnpm ci:check` runs `nx affected -t ... build`, which uses the production configuration by default. A PR that adds an `i18n` marker without updating `messages.fr.xlf` fails its build job → merge blocked.
- The default build configuration is unchanged (`production`), so no other CI plumbing is needed.

## Verification

Locally, temporarily inserting an unmarked-in-fr string:

```html
<span i18n="@@sanity.test.missingTranslation">A string with no FR translation</span>
```

then running `pnpm exec nx build portal-shell --configuration=production` produces:

```
ERROR: No translation found for "sanity.test.missingTranslation"
       ("A string with no FR translation").
Application bundle generation failed.
NX   Running target build for project portal-shell failed
```

…with a non-zero exit code. The patch was reverted before commit.

## Test plan

- [x] `pnpm exec nx run-many -t lint test build --projects=portal-shell` — green (40 / 40 specs, build still passes on the current state).
- [x] Sanity-check that the gate actually fires when a translation is missing (above).
- [ ] CI: the next PR that adds an `i18n` marker without updating `messages.fr.xlf` should fail at the `build` step with the explicit error.

## What this PR explicitly does NOT do

- Catch **unmarked** source strings (e.g. a developer who forgets to add `i18n="@@..."` to a new template line). That would need a custom ESLint rule against the Angular template AST — a separate, smaller scope mentioned in ADR-0019 as a future refinement.
- Localise editorial / CMS content. Editorial copy comes from the BFF already localised; this gate covers only the developer-owned UI strings baked at build time.
- Add a similar gate to the i18n extraction (we could fail the build if `nx extract-i18n` produces diffs against the committed `messages.xlf`, but we don't commit `messages.xlf` today).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #98

apps/portal-shell/project.json

@@ -38,6 +38,7 @@
       "configurations": {
         "production": {
           "localize": ["en", "fr"],
+          "i18nMissingTranslation": "error",
           "budgets": [
             {
               "type": "initial",