fix: increase markdown transform word limit to 2000 + i18n errors

Same fix as reformulate — raise limit from 500 to 2000 and use i18n
error keys instead of hardcoded English messages.

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

memento-note/app/api/ai/transform-markdown/route.ts

@@ -22,14 +22,14 @@ export async function POST(request: NextRequest) {
     const wordCount = text.split(/\s+/).length
     if (wordCount < 10) {
       return NextResponse.json(
-        { error: 'Text must have at least 10 words to transform' },
+        { errorKey: 'ai.wordCountMin', params: { min: 10, current: wordCount } },
         { status: 400 }
       )
     }
 
-    if (wordCount > 500) {
+    if (wordCount > 2000) {
       return NextResponse.json(
-        { error: 'Text must have maximum 500 words to transform' },
+        { errorKey: 'ai.wordCountMax', params: { max: 2000, current: wordCount } },
         { status: 400 }
       )
     }