Sit the transcript on the bottom when it does not fill the screen

A session whose loaded rows were shorter than the viewport drew them
against the top of the list, leaving a gap between the newest message and
the box you type in -- and nothing to scroll, because there was no
overflow. Opening the keyboard shrank the viewport enough for the content
to overflow it and the list snapped down, which made a placement fault
look like a scrolling one.

`reverseLayout` defaults the arrangement to `Bottom` on its own, but
naming `spacedBy` replaces that default with `spacedBy`'s own, which is
`Top`. The arrangement is only consulted when the content does not fill
the viewport, which is why this sat here since 2026-08-28 without being
seen: it needs a session that loads less than a screenful, and a page of
tool calls collapsing to one "Called 80 tools" row is how a long
conversation manages that.

app/androidApp/src/main/kotlin/com/example/aiapp/SessionScreen.kt

@@ -1057,7 +1057,16 @@ fun SessionScreen(
                 reverseLayout = true,
                 modifier = Modifier.fillMaxSize(),
                 contentPadding = PaddingValues(16.dp),
-                verticalArrangement = Arrangement.spacedBy(8.dp),
+                // Bottom, and it has to be said: `reverseLayout` defaults the arrangement to
+                // `Bottom` on its own, but naming `spacedBy` replaces that default with
+                // `spacedBy`'s own, which is `Top`. The arrangement is what places the content
+                // when there is less of it than the viewport -- so a session whose loaded rows
+                // did not fill the screen drew them against the *top*, leaving a gap between the
+                // newest message and the box you type in, and no room to scroll the gap away.
+                // Opening the keyboard shrank the viewport enough for the content to overflow it
+                // and the list snapped down, which is what made it look like a scrolling fault
+                // rather than a placement one.
+                verticalArrangement = Arrangement.spacedBy(8.dp, Alignment.Bottom),
             ) {
                 // The last thing in the transcript, because that is where
                 // they are in the session's reading of events: after