fix: replace raw ai_consent_required code with translatable message and consent prompt

- Centralize AI consent 403 responses in server-consent helpers
- Return human-readable message + code + i18n errorKey from all AI APIs
- Update dashboard and AI clients to detect code/errorKey and translate
- Add consent.ai.requiredToast i18n keys (fr/en)

memento-note/app/api/chat/route.ts

@@ -6,7 +6,7 @@ import { getChatProvider } from '@/lib/ai/factory'
 import { semanticSearchService } from '@/lib/ai/services/semantic-search.service'
 import { prisma } from '@/lib/prisma'
 import { auth } from '@/auth'
-import { hasUserAiConsent } from '@/lib/consent/server-consent'
+import { hasUserAiConsent, aiConsentForbiddenResponse } from '@/lib/consent/server-consent'
 import { loadTranslations, getTranslationValue, SupportedLanguage } from '@/lib/i18n'
 import { toolRegistry } from '@/lib/ai/tools'
 import { reserveAiUsageOrThrow, handleQuotaHttpError } from '@/lib/ai-quota'
@@ -65,10 +65,7 @@ export async function POST(req: Request) {
 
   // GDPR AI Consent check
   if (!(await hasUserAiConsent())) {
-    return new Response(JSON.stringify({ error: 'ai_consent_required' }), {
-      status: 403,
-      headers: { 'Content-Type': 'application/json' },
-    })
+    return aiConsentForbiddenResponse()
   }
 
   // 1.5 Quota check (per-provider BYOK bypass — only when BYOK will be used for resolved provider)