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

webpack-cli build --node-env=production failed on two issues that
ts-jest's --transpile-only didn't surface:

1. portal-bff TS2454 in user-scopes.service.ts:221 — TypeScript
   couldn't see the switch on `kind` as exhaustive, so `let exists:
   boolean` was flagged as "used before assigned". The narrowing on
   VALUE_BEARING_KINDS.has(kind) doesn't propagate through Set.has
   (only Array.includes / type-guards do). Initialise `exists =
   false` so it's definitely assigned regardless of switch path;
   semantically safe — a non-value-bearing kind that reaches the
   switch (shouldn't happen by construction) leaves exists=false and
   the next `if (!exists)` throws the same friendly "does not match"
   message.

2. portal-admin missing translation: route.user-scopes.title was
   declared in app.routes.ts but never added to messages.fr.xlf.
   @angular/localize build target requires every $localize id to
   have a target in the active locale. Added the trans-unit for the
   FR locale ("Périmètres utilisateur — Administration APF Portal").

Local: nx run-many -t build --projects=portal-bff,portal-admin
passes on both.

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 =