front: config env, error interceptor fonctionnel, DSFR en deps, fix build

- src/environments/{environment,environment.prod}.ts + fileReplacements :
  baseUrl API configurable au lieu de '/api' hardcodé dans les services
- ErrorService + errorInterceptor (HttpInterceptorFn) ; app.config passe à
  withInterceptors([...]) (remplace withInterceptorsFromDi legacy)
- @gouvfr/dsfr déplacé en dependencies (chargé au runtime)
- api.service: extrait ValidateResponse (dédup du type inline)
- fix: formValues passe en protected (utilisé dans le template -> build NG1
  cassé, le conteneur servait un ancien bundle)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

front-public/angular.json

@@ -57,7 +57,13 @@
                 { "type": "initial", "maximumWarning": "500kB", "maximumError": "1MB" },
                 { "type": "anyComponentStyle", "maximumWarning": "4kB", "maximumError": "8kB" }
               ],
-              "outputHashing": "all"
+              "outputHashing": "all",
+              "fileReplacements": [
+                {
+                  "replace": "src/environments/environment.ts",
+                  "with": "src/environments/environment.prod.ts"
+                }
+              ]
             },
             "development": {
               "optimization": false,