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

README.md

@@ -47,15 +47,17 @@ halves, so the root `make check` fails if either one is broken. We provide:
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
 - `script/projectname` — print the project name (used for the Docker image tag)
-- `script/test` — run `script/frontend-test`, then the backend's Go tests, each
-  under its own 30-second timeout
+- `script/test` — run `script/frontend-test`, then `backend/script/test`, the
+  backend's Go tests with the race detector and coverage
 - `script/lint` — run eslint, then golangci-lint, both in Docker, by building
   the `frontend-lint` and `lint` stages of `Dockerfile` without the cache
 - `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`
 - `script/fmt-check` — check formatting (read-only): prettier, then gofmt
 - `script/check` — run test, lint, and fmt-check
 - `script/frontend-test` — run the unit tests in `test/unit/` with Node's
-  built-in test runner, then the production build
+  built-in test runner, through the `test` script in `package.json`, and if any
+  fails, run them again listing every test, and fail; then the production build.
+  Each run has a 30-second timeout
 - `script/frontend-lint` — run eslint with the rules in `eslint.config.js`; it
   runs inside the `frontend-lint` stage of `Dockerfile`, which `make lint`
   builds
@@ -267,7 +269,6 @@ properties.
 
 ## TODO
 
-- Add unit tests
 - Add configurable host list (environment variable or config file)
 - Add latency history export (CSV/JSON)
 - Add notification/alert when status changes to DEGRADED