Clock errors no longer stop probing; legend reads >=500ms (closes #149, closes #152)

The header's clocks start last in init, after the rounds, and an error
formatting them, as in a browser that rejects the "shortGeneric" time
zone name, leaves both clocks blank instead of stopping the page before
its first round. updateClocks moved out of init so a unit test can call
it.

The footer legend's red entry reads >=500ms, as README.md and
CONFIG.latencyColors have it. buildUI is exported so a unit test can draw
the page and check the legend against every entry of
CONFIG.latencyColors.

Model: opus-5-5

TODO.md

@@ -24,14 +24,14 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 - 2026-10-18: a browser that cannot format the header's clocks still probes
   ([#149](https://git.eeqj.de/sneak/netwatch/issues/149)). The clocks name the
   local time zone through `Intl.DateTimeFormat`'s `shortGeneric` option, which
-  browsers from before 2022 reject with an error; it came before the first round
-  started, so such a browser never checked a target. The clocks now start last,
-  after the rounds, and an error formatting them leaves both blank. The footer's
-  color legend reads `>=500ms` for red, where it read `>500ms`, so 500 ms, which
-  is red, is in one of its colors
+  Chrome before 95, Firefox before 91 and Safari before 15.4 reject with an
+  error; it came before the first round started, so such a browser never checked
+  a target. The clocks now start last, after the rounds, and an error formatting
+  them leaves both blank. The footer's color legend reads `>=500ms` for red,
+  where it read `>500ms`, so 500 ms, which is red, is in one of its colors
   ([#152](https://git.eeqj.de/sneak/netwatch/issues/152)). Unit tests check the
   clocks with a working `Intl.DateTimeFormat` and with one that rejects
-  `shortGeneric`, and draw the page and check the legend's limits against
+  `shortGeneric`, and draw the page and check the legend against every entry of
   `CONFIG.latencyColors`
 - 2026-10-18: from a trusted proxy, the backend takes the client address from
   the right-most `X-Forwarded-For` entry that is not in `TRUSTED_PROXIES`

test/unit/main.test.js

@@ -588,9 +588,12 @@ test("a check that timed out or found its target unreachable has a grey figure a
 // buildUI draws the page into #app, here a plain object. Each entry in the
 // footer's color legend is a colored dot and the latencies it stands for,
 // with < and > written as &lt; and &gt;. A latency takes the color of the
-// first entry in CONFIG.latencyColors whose limit it is below, so red, the
-// last, starts at orange's limit.
-test("the footer's color legend gives the latency limits in CONFIG.latencyColors, each latency in one color", (t) => {
+// first entry in CONFIG.latencyColors whose limit it is below, so the
+// legend has, for each of those entries in order, a dot of its color (a
+// bg- class where CONFIG.latencyColors has a text- one) and the latencies
+// below its limit, or, for the last, which has no limit, the latencies
+// from the limit before it up. A gray dot for offline comes after them.
+test("the footer's color legend has each entry in CONFIG.latencyColors, with its color and limit, then offline", (t) => {
     const app = {};
     t.mock.method(globalThis.document, "getElementById", () => app);
     buildUI(new AppState([]));
@@ -598,15 +601,14 @@ test("the footer's color legend gives the latency limits in CONFIG.latencyColors
     const legend = [
         ...app.innerHTML.matchAll(/(bg-\w+-500) [^"]*"><\/span>(\S+)/g),
     ].map(([, dot, latencies]) => [dot, latencies]);
-    const limits = CONFIG.latencyColors.map((color) => color.below);
-    assert.deepEqual(legend, [
-        ["bg-green-500", `&lt;${limits[0]}ms`],
-        ["bg-lime-500", `&lt;${limits[1]}ms`],
-        ["bg-yellow-500", `&lt;${limits[2]}ms`],
-        ["bg-orange-500", `&lt;${limits[3]}ms`],
-        ["bg-red-500", `&gt;=${limits[3]}ms`],
-        ["bg-gray-500", "offline"],
+    const colors = CONFIG.latencyColors;
+    const expected = colors.map((color, i) => [
+        color.className.replace("text-", "bg-"),
+        color.below === Infinity
+            ? `&gt;=${colors[i - 1].below}ms`
+            : `&lt;${color.below}ms`,
     ]);
+    assert.deepEqual(legend, [...expected, ["bg-gray-500", "offline"]]);
 });
 
 // A target whose checks, in turn, answered after each of latencies ms, or,