feat(timeline): hold back unstarted events, and say where they begin

Two halves of one reading. The board answers "how does the time I am in
lay out?", and plotting every lane's next patch stretched the window
weeks past today and squeezed the running bars down to nothing — so it
holds them back, and holds them back itself rather than being handed a
shorter list, because the control has to say *how many*. A board that
quietly withheld a third of the schedule is indistinguishable from a
quiet fortnight, which is the wrong thing for this app to imply by
accident. A board with nothing left running says so and points at the
control instead of reading as an empty calendar.

Switched on, a bar drawn right of the now rule is only implicitly in the
future, and implicitly is not a standard we hold anywhere else a date is
involved. Gacha schedules are not a smooth stream of starts either — a
patch ships six things at once — so the unit is the clump: a dashed rule
per group of starts, labelled in words. Clumps merge by distance on
screen rather than by calendar, since a week apart is one mark at six
pixels a day and two at a hundred and eight, and a merged one states the
span it covers rather than claiming a date the board is not ruling at.

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

src/client/App.tsx

@@ -548,6 +548,10 @@ export function App() {
             onZoom={(timelineDayWidth) => update({ timelineDayWidth })}
             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.
+            showUpcoming={prefs.timelineUpcoming}
+            onShowUpcoming={(timelineUpcoming) => update({ timelineUpcoming })}
             onOpen={setOpenId}
             isDone={isDone}
           />