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/lib/ai/services/paragraph-refactor.service.ts

@@ -56,7 +56,7 @@ export const REFACTOR_OPTIONS: RefactorOption[] = [
 export class ParagraphRefactorService {
   private languageDetection: LanguageDetectionService
   private readonly MIN_WORDS = 10
-  private readonly MAX_WORDS = 500
+  private readonly MAX_WORDS = 2000
 
   constructor() {
     this.languageDetection = new LanguageDetectionService()
@@ -288,20 +288,24 @@ CRITICAL: Respond ONLY with the refactored text in ${language}. No explanations,
   /**
    * Validate that text is within acceptable word count range
    */
-  validateWordCount(content: string): { valid: boolean; error?: string } {
+  validateWordCount(content: string): { valid: boolean; error?: string; errorKey?: string; params?: Record<string, number> } {
     const wordCount = content.split(/\s+/).length
 
     if (wordCount < this.MIN_WORDS) {
       return {
         valid: false,
-        error: `Please select at least ${this.MIN_WORDS} words to reformulate (currently ${wordCount} words)`
+        errorKey: 'ai.wordCountMin',
+        params: { min: this.MIN_WORDS, current: wordCount },
+        error: `min:${this.MIN_WORDS}:current:${wordCount}`
       }
     }
 
     if (wordCount > this.MAX_WORDS) {
       return {
         valid: false,
-        error: `Please select at most ${this.MAX_WORDS} words to reformulate (currently ${wordCount} words)`
+        errorKey: 'ai.wordCountMax',
+        params: { max: this.MAX_WORDS, current: wordCount },
+        error: `max:${this.MAX_WORDS}:current:${wordCount}`
       }
     }