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 and one of only answers, 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

package.json

@@ -6,7 +6,8 @@
     "scripts": {
         "dev": "vite",
         "build": "vite build",
-        "preview": "vite preview"
+        "preview": "vite preview",
+        "test": "node --test test/unit/*.test.js"
     },
     "license": "MIT",
     "devDependencies": {