library: bound the detail scroll restore and cover grids

Gate the restore on the caller's content being ready instead of an item
count that can include only the header, clear it after clamping to the
available rows, and include offset-only positions. Apply the same saver
to the artist grid so a process-death restore does not open at the top.

app/src/test/kotlin/com/subtracks/ui/components/HeroDetailScaffoldRestoreTest.kt

@@ -46,6 +46,7 @@ class HeroDetailScaffoldRestoreTest {
                             onBack = {},
                             header = { _, _ -> Box(Modifier.height(200.dp)) },
                             content = { rowModifier -> rowItems(rows, rowModifier) },
+                            contentReady = rows > 0,
                         )
                     }
                 }