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/state/lens.ts

@@ -71,6 +71,31 @@ export function firstToExpire<T extends Row>(rows: readonly T[]): T | null {
   return nextToExpire(rows, 1)[0] ?? null;
 }
 
+/**
+ * How long the deadline *behind* the closest one has left, or null when there
+ * is not a second dated deadline to report.
+ *
+ * The "Running now" header uses this to say what falls due after the row at the
+ * top. It asked the list for `rows[1]` before, which was wrong twice over and
+ * in the two ways this module exists to prevent.
+ *
+ * The list arrives sorted by whatever mode the reader chose, so under "doing
+ * first" `rows[1]` is the second thing they are partway through — the same
+ * mistake `firstToExpire` was written to stop the headline making, one row
+ * further down. And an unannounced end has no time remaining at all, so the
+ * old `?? 0` handed `formatRemaining` a zero and the header read "next after
+ * this ends in ended": the `endsAt: null` rule turned into a claim that
+ * something expired, which is exactly backwards.
+ *
+ * So it reads the second *dated* deadline and returns null rather than a
+ * stand-in, and the caller omits the line when there is nothing to say.
+ */
+export function followingDeadlineMs<T extends Row>(
+  rows: readonly T[],
+): number | null {
+  return nextToExpire(rows, 2)[1]?.clock.msRemaining ?? null;
+}
+
 /**
  * The focused game, if it is still a game the reader can see.
  *