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/app/api/ai/reformulate/route.ts

@@ -42,7 +42,11 @@ export async function POST(request: NextRequest) {
     // Validate word count
     const validation = paragraphRefactorService.validateWordCount(text)
     if (!validation.valid) {
-      return NextResponse.json({ error: validation.error }, { status: 400 })
+      return NextResponse.json({
+        error: validation.error,
+        errorKey: validation.errorKey,
+        params: validation.params,
+      }, { status: 400 })
     }
 
     // Use the ParagraphRefactorService