fix: increase reformulate word limit to 2000 + i18n error messages

- Raise MAX_WORDS from 500 to 2000 for text reformulation
- Error messages now use i18n keys (ai.wordCountMin/Max) instead of
  hardcoded English strings
- Client translates server errors using the user's UI language

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/components/contextual-ai-chat.tsx

@@ -205,7 +205,13 @@ export function ContextualAIChat({
         body: JSON.stringify(action.body(noteContent, undefined, language)),
       })
       const data = await res.json()
-      if (!res.ok) throw new Error(data.error || t('ai.genericError'))
+      if (!res.ok) {
+        // Use i18n key if provided by the server
+        if (data.errorKey) {
+          throw new Error(t(data.errorKey, data.params || {}))
+        }
+        throw new Error(data.error || t('ai.genericError'))
+      }
       const result = data[action.resultKey] || ''
       setActionPreview({ label: t(action.i18nKey), text: result })
     } catch (e: any) {