feat(timeline): offer both readings of a board with the future on it

"In their own group" is the shape of a patch — what is on now, and what
is queued behind it. "Mixed in" is the question a Gantt chart exists for:
what runs out first, whether or not it has opened. On real data that is
not a cosmetic difference — mixed, five events currently running sort
below an Arknights rerun that has not started, because it closes before
they do.

Which is why it is a pair of pills and not a checkbox: neither answer is
the absence of the other, and "mixed in" is a different order rather than
the heading switched off. It sits under the row that puts the future on
the board at all, and only while that row is ticked — a choice about
arranging unstarted events is unanswerable with none of them on screen.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/App.tsx

@@ -552,6 +552,7 @@ export function App() {
             // shorter list, so it can say how many are waiting when there is
             // nothing else left to draw. The switch is in settings.
             showUpcoming={prefs.timelineUpcoming}
+            splitUpcoming={prefs.timelineSplitUpcoming}
             onOpen={setOpenId}
             isDone={isDone}
           />