fix: stop the running-now header counting down to "ended"

The section header said "next after this ends in {live[1]}", which got the
question wrong in both of the ways lens.ts exists to prevent.

An event whose end was never announced has no time remaining, and the `?? 0`
that stood in for it made `formatRemaining` return its expiry string — so a
second row with `endsAt: null` had the header reading "next after this ends in
ended". That is the one rule this product is built on, inverted: an unknown end
announced as an expiry.

And `live[1]` is the second *row*, not the second deadline. The list is sorted
by whatever mode the reader picked, so under "doing first" the header named
whatever they were second-most partway through — the same mistake firstToExpire
was written to keep out of the headline, one row further down.

`followingDeadlineMs` asks the deadlines instead and returns null when there is
no second dated end, and the header then says nothing at all.

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

src/client/App.tsx

@@ -24,6 +24,7 @@ import { compareRows, SORT_MODES, type Activity, type SortMode } from "./state/s
 import {
   advanceFocus,
   countByGame,
+  followingDeadlineMs,
   nextToExpire,
   outstanding,
   resolveFocus,
@@ -300,6 +301,15 @@ export function App() {
 
   const live = visible.filter((r) => r.clock.live);
   const upcoming = visible.filter((r) => r.clock.upcoming);
+  /**
+   * What falls due after the row at the top of "Running now".
+   *
+   * Read off the deadlines rather than off the list, because the list is in
+   * whichever order the reader chose — see `followingDeadlineMs`. Null when
+   * there is no second dated end, and the header then says nothing rather than
+   * counting down to a placeholder.
+   */
+  const following = followingDeadlineMs(live);
   /**
    * The unstarted events the checklist actually lists.
    *
@@ -512,11 +522,9 @@ export function App() {
               legend
               title="Running now"
               hint={
-                live.length > 1
-                  ? `next after this ends in ${formatRemaining(
-                      live[1]?.clock.msRemaining ?? 0,
-                    )}`
-                  : undefined
+                following === null
+                  ? undefined
+                  : `next after this ends in ${formatRemaining(following)}`
               }
               action={
                 visible.length > 1 ? (