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/ai/echo/fusion/route.ts

@@ -3,7 +3,7 @@ import { auth } from '@/auth'
 import { getChatProvider } from '@/lib/ai/factory'
 import { getSystemConfig } from '@/lib/config'
 import prisma from '@/lib/prisma'
-import { hasUserAiConsent } from '@/lib/consent/server-consent'
+import { hasUserAiConsent, aiConsentForbiddenResponse } from '@/lib/consent/server-consent'
 import { withAiQuota, handleQuotaHttpError } from '@/lib/ai-quota'
 
 /**
@@ -22,7 +22,7 @@ export async function POST(req: NextRequest) {
     }
 
     if (!(await hasUserAiConsent())) {
-      return NextResponse.json({ error: 'ai_consent_required' }, { status: 403 })
+      return aiConsentForbiddenResponse()
     }
 
     const body = await req.json()