fix: drop "No article selected" flash on back from article

ClearArticle nulls state before exit transition ends, so the outgoing screen briefly rendered the null-branch fallback in the title bar. Render empty title/content for null state; remove article_no_selected.

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

@@ -196,16 +196,7 @@ fun ArticleContent(
             }
         }
         null -> {
-            Column(
-                modifier = modifier.fillMaxSize(),
-                horizontalAlignment = Alignment.CenterHorizontally,
-                verticalArrangement = Arrangement.Center
-            ) {
-                Text(
-                    text = stringResource(R.string.article_no_selected),
-                    style = MaterialTheme.typography.bodyLarge
-                )
-            }
+            Box(modifier = modifier.fillMaxSize())
         }
     }
 }
@@ -221,7 +212,7 @@ fun ArticleTitleBar(
         is ArticleState.Loading -> articleState.title
         is ArticleState.Success -> articleState.title
         is ArticleState.Error -> articleState.title
-        null -> stringResource(R.string.article_no_selected)
+        null -> ""
     }
 
     val backIcon = painterResource(R.drawable.ic_arrow_back)