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. Rounds never overlap: a
round gives up the last round's checks if they are still waiting, which
happens only when a round starts early, after an interval change or
when the recovery probe finds a target answering. The probe still
checks every half second, giving up its previous checks, so they do not
pile up.

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

Model: opus-5-5

script/frontend-test

@@ -1,13 +1,14 @@
 #!/bin/sh
-# script/frontend-test: run the frontend test suite. The frontend has no
-# unit tests; the production build serves as the test (fails on broken
-# code).
+# script/frontend-test: run the frontend test suite: the unit tests in
+# test/unit/ with Node's built-in test runner, then the production
+# build, which fails on broken code.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
+    timeout 30 node --test test/unit/*.test.js
     timeout 30 yarn build
 }