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/components/fusion-modal.tsx

@@ -69,7 +69,7 @@ export function FusionModal({
       const data = await res.json()
       
       if (!res.ok) {
-        throw new Error(data.error || 'Failed to generate fusion')
+        throw new Error(data.errorKey ? t(data.errorKey) : (data.error || 'Failed to generate fusion'))
       }
 
       if (!data.fusedNote) {