Frontend unit tests cover durations, colours, statistics and health (closes #21)

New table-driven tests in test/unit/main.test.js: humanDuration, the
figure and sparkline colours either side of each latency boundary, a
target's min, max, average and median over an empty history, an
all-unreachable one, a mixed one, one of only answers and one whose
answers have different numbers of digits, and the four health states
either side of their thresholds, with targets found unreachable counted
as timed out. src/main.js exports the four names they need.

package.json gains a test script, which script/frontend-test runs with
the dot reporter and, if a test fails, again with the spec reporter
before failing. NODE_OPTIONS picks the reporter, since yarn appends its
arguments after the test files.

Model: opus-5-5

TODO.md

@@ -23,6 +23,16 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-10-03: the frontend's unit tests cover what the page computes (issue
+  #21): `humanDuration`, the latency colours of a figure and of a sparkline
+  either side of each boundary, a target's min, max, average and median latency
+  over an empty history, an all-unreachable one and a mixed one, and each of the
+  four health states either side of its thresholds. `package.json` has a `test`
+  script, so `yarn run test` and `npm run test` run them, and
+  `script/frontend-test` runs it: quietly, and if a test fails, again with every
+  test listed, and then fails. `src/main.js` now exports `humanDuration`,
+  `HostState`, `latencyClass` and `latencyHex` for the tests; nothing it does
+  changed
 - 2026-10-03: the backend's logs are one stream (issue #27): fx logs its own
   steps of starting and stopping through the backend's logger, so off a terminal
   every line the backend's own logger and fx write is JSON, where fx used to