feat: put the stacking toggle on the board

The board's header said "One lane per game" and left it at that; now it
offers the other stacking too, in the same place and the same pill shape
the list's sort control uses — you reach for it while looking at the
board, not in settings.

Merged, there is no lane heading to say whose event a bar is, and hue
alone cannot answer that once every game shares one stack. So each bar
carries its game's short name, and its tooltip carries the full one. A
bar under 96px has no room for a tag and a title both, and a chopped
game name reads as a broken word — those keep the title and say the game
in the tooltip.

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

src/client/App.tsx

@@ -535,6 +535,8 @@ export function App() {
             // reader edited, and a board one pixel wide is not a preference.
             dayWidth={snapDayWidth(prefs.timelineDayWidth)}
             onZoom={(timelineDayWidth) => update({ timelineDayWidth })}
+            group={prefs.timelineGroup}
+            onGroup={(timelineGroup) => update({ timelineGroup })}
             onOpen={setOpenId}
             isDone={isDone}
           />