fix(admin): unbreak ci:check on PR2b (TS2454 + missing fr translation) (#237)

## Summary

Follow-up fix on [#236](#236) (ADR-0026 PR 2b). Two issues that `nx run-many -t build` exposed but `nx run-many -t test` masked:

1. **TS2454 in `user-scopes.service.ts`** — webpack's strict-mode build flagged `let exists: boolean` as "used before assigned". jest's `--transpile-only` doesn't enforce definite-assignment so it slipped through local checks.
2. **Missing FR translation** for `route.user-scopes.title` — Angular's `@angular/localize` build target requires every `$localize` ID to have a target in `messages.fr.xlf`. The route I added in #236 referenced the ID but I didn't add the trans-unit.

## What lands

| File | Change |
| --- | --- |
| `apps/portal-bff/src/admin/user-scopes.service.ts` | Initialise `let exists = false` instead of `let exists: boolean`. The `VALUE_BEARING_KINDS.has(kind)` check earlier in the function does narrow `kind` semantically, but `Set.has` doesn't propagate type narrowing the way `Array.includes` or a custom type guard would. Initialising to `false` is the smallest fix; the next `if (!exists)` still throws the friendly "does not match" message in the (unreachable-by-construction) fallback. |
| `apps/portal-admin/src/locale/messages.fr.xlf` | New `<trans-unit id="route.user-scopes.title">` — FR target `Périmètres utilisateur — Administration APF Portal`. |

## Why these failed locally

- **TS2454**: jest runs with `--transpile-only` so it skips definite-assignment analysis (and most type-checking). Webpack's production build runs full `tsc` and catches it.
- **Missing translation**: the dev server `pnpm nx serve portal-admin` doesn't run the localize pass; only `--configuration=production` does. My local `nx test portal-admin` runs in JIT mode against the source locale, so the missing FR target slid through.

For the future: `pnpm nx run-many -t build --projects=portal-bff,portal-admin` would have caught both pre-PR. Worth a `pnpm ci:check:fast` shorthand that runs a subset of the CI gates locally before push — but that's a separate PR.

## Test plan

- [x] `pnpm exec nx run-many -t build --projects=portal-bff,portal-admin` — both pass.
- [x] No application logic changed in either fix (initialise vs declare; add a missing translation row).
- [ ] CI green on this PR.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #237

apps/portal-admin/src/locale/messages.fr.xlf

@@ -21,6 +21,10 @@
         <source>Users — APF Portal Admin</source>
         <target>Utilisateurs — Administration APF Portal</target>
       </trans-unit>
+      <trans-unit id="route.user-scopes.title" datatype="html">
+        <source>User scopes — APF Portal Admin</source>
+        <target>Périmètres utilisateur — Administration APF Portal</target>
+      </trans-unit>
       <trans-unit id="route.profile.title" datatype="html">
         <source>Profile — APF Portal Admin</source>
         <target>Profil — Administration APF Portal</target>

apps/portal-bff/src/admin/user-scopes.service.ts

@@ -194,7 +194,15 @@ export class UserScopesService {
     // ADR-0026 PR 1's "no FK on UserScope.value" rationale lets the
     // value persist even after the target is decommissioned, so we
     // validate only at the write path.
-    let exists: boolean;
+    //
+    // Initialise `exists = false` so TypeScript sees it as definitely
+    // assigned regardless of how the switch lands (the type-system
+    // can't see that `VALUE_BEARING_KINDS.has` already narrowed
+    // `kind` to the three branches). If somehow a non-value-bearing
+    // kind reached this code, the false default makes the next `if`
+    // throw with the same friendly "does not match" message — safe
+    // by construction.
+    let exists = false;
     switch (kind) {
       case 'etablissement':
         exists =