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/locales/en.json

@@ -3806,7 +3806,8 @@
       "revoked": "Consent not granted",
       "revokedToast": "AI processing consent successfully revoked.",
       "complianceBadge": "GDPR compliance",
-      "auditFailed": "Could not record your consent. Please try again."
+      "auditFailed": "Could not record your consent. Please try again.",
+      "requiredToast": "Enable AI processing in Settings → AI to use this feature."
     }
   },
   "account": {