Target check timeout is 80% of the refresh interval (closes #78)

Each target check now times out after 80% of the refresh interval, 24
seconds at 30 seconds, where it was capped at 3 seconds, so slow, far
targets are recorded with their real time. A round asked for while the
last one's checks are still waiting, after an interval change or by the
recovery probe, is skipped, so rounds never overlap; the recovery probe
starts no new checks while its last ones wait.

The frontend has its first unit tests, run by script/frontend-test with
Node's built-in test runner. index.html now links src/styles.css, which
src/main.js imported, since Node cannot import CSS.

Model: opus-5-5

TODO.md

@@ -23,6 +23,13 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: each target check times out after 80% of the refresh interval
+  (issue #78), 24 seconds at 30 seconds, where it was capped at 3 seconds. A
+  round due while the last one's checks are still waiting is skipped, so rounds
+  never overlap, and the recovery probe starts no new checks while its last ones
+  are waiting. The frontend has its first unit tests, run by
+  `script/frontend-test` with Node's built-in test runner; for them,
+  `index.html` now links `src/styles.css`, which `src/main.js` used to import
 - 2026-09-29: the container sets up its own data directory (issue #75):
   `bin/entrypoint.sh`, still as root, creates `DATA_DIR` if missing and gives it
   and `/data` to the `netwatch` user with mode 750 before starting the backend