fix(article): stabilize baseTextStyle across recompositions

TextStyle.copy() returned a new instance on every recomposition, so the
remember() cache for the rendered blocks never hit. A text selection re-ran
block merging and recreated all article text nodes, rebuilding their
StaticLayouts repeatedly. Hoist the theme values and remember the style.

app/src/main/java/com/example/research/ui/article/ArticleScreen.kt

@@ -118,13 +118,17 @@ fun ArticleContent(
         }
         is ArticleState.Success -> {
             val articleSaveKey = "${articleState.dictionaryPath}:${articleState.title}"
-            val baseTextStyle = MaterialTheme.typography.bodyLarge.copy(
-                color = MaterialTheme.colorScheme.onSurface,
+            val colorScheme = MaterialTheme.colorScheme
+            val typography = MaterialTheme.typography
+            val baseTextStyle = remember(colorScheme, typography) {
+                typography.bodyLarge.copy(
+                    color = colorScheme.onSurface,
                     lineHeightStyle = LineHeightStyle(
                         alignment = LineHeightStyle.Alignment.Proportional,
                         trim = LineHeightStyle.Trim.Both,
                     ),
                 )
+            }
             val indentStep = 16.dp
 
             // Use scroll position from ArticleState if available (for back navigation)