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/flashcards/flashcard-generate-dialog.tsx

@@ -64,11 +64,7 @@ export function FlashcardGenerateDialog({
       })
       const data = await res.json()
       if (!res.ok) {
-        if (data.errorKey) {
-          toast.error(t(data.errorKey) || data.error)
-        } else {
-          toast.error(data.error || t('flashcards.generateFailed'))
-        }
+        toast.error(data.errorKey ? t(data.errorKey) : (data.error || t('flashcards.generateFailed')))
         return
       }
       setCards(data.cards || [])