refactor(timeline): move the unstarted-events switch into settings

It shipped as a pill in the board's own header, beside the stacking and
scale controls. That is the wrong company: those two reshape what is
already on the board, which is why they are reached for while reading it,
and this one decides what is on it at all — the same question
"Show events I've finished" and "Show events I'm ignoring" answer, from
the panel where both of those live.

It is the only one of the three scoped to a single view, so the row says
so instead of reading as a promise about the whole app. The board loses
the count the pill carried; with nothing left running it still says how
many are waiting and names the setting, and the page header has said
"N live · N upcoming" all along.

Controls had no tests at all, so the three view filters get some: what
each checkbox is bound to is invisible in a diff and obvious only to the
reader it happens to.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/client/App.tsx

@@ -549,9 +549,9 @@ export function App() {
             group={prefs.timelineGroup}
             onGroup={(timelineGroup) => update({ timelineGroup })}
             // The board holds these back itself rather than being handed a
-            // shorter list, so its own control can say how many are waiting.
+            // 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}
-            onShowUpcoming={(timelineUpcoming) => update({ timelineUpcoming })}
             onOpen={setOpenId}
             isDone={isDone}
           />