Summary reads --/-- after an interval change, not 0/26 in red (closes #140)

The interval menu resets every host to pending and updates the summary,
which counted the WAN hosts with a latency recorded, none, and showed
0/26 in red until the second round after the change ended. While every
WAN host still waits for its first check, the Reachable slot now reads
--/-- in white, as at page load. updateSummary is exported for a unit
test that resets the hosts as the interval menu does; a second test
checks that the slot still reads 0/26 in red once every check failed.

Model: opus-5-5

TODO.md

@@ -21,6 +21,14 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-18: after an interval change, the summary's Reachable slot reads
+  `--/--` in white, as at page load, until the second round since the change
+  ends ([#140](https://git.eeqj.de/sneak/netwatch/issues/140)). It used to read
+  0/26 in red from the change on, as no WAN host had a check recorded, for up to
+  two intervals with the network fine. The summary update now writes `--/--`
+  while every WAN host still waits for its first check. A unit test resets the
+  hosts as the interval menu does and checks the slot; another checks that it
+  reads 0/26 in red once every WAN host's check has failed
 - 2026-10-14: stopping the server waits for a report file write already under
   way, the periodic flush's or the one `Append` starts when 10 MiB are waiting,
   before the final flush