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/pages/article/widgets/opus_content.dart

@@ -151,7 +151,7 @@ class OpusContent extends StatelessWidget {
       style: _getStyle(
         word?.style,
         color ?? defaultColor,
-        word?.fontSize,
+        word?.effectiveFontSize,
       ),
     );
   }