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 empty, all-unreachable and
mixed histories, and the four health states either side of their
thresholds. 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