fix(article): fix Opus article body font size too small (#1897)

New Opus format uses font_level string instead of numeric font_size,
causing font size to fall back to Flutter theme default (14px) instead
of the expected 16px. Added Word.effectiveFontSize: small→13px,
others→16px (consistent with legacy HTML article format).

lib/models/dynamics/article_content_model.dart

@@ -134,6 +134,13 @@ class Word {
           );
     fontLevel = json['font_level'];
   }
+
+  // font_level 映射处理：
+  //   "small"   → 13px
+  //   "regular" → 16px（与旧版 HTML 专栏基准一致）
+  //   其余/null → 同 regular
+  double get effectiveFontSize =>
+      fontSize ?? (fontLevel == 'small' ? 13.0 : 16.0);
 }
 
 class Style {