Health: count only flows the engine acts on, format durations with their unit

The "running" tile counted paused and invalid flows as running, so it read
"5/5" beside "1 flow(s) cannot run". Its note is now additive rather than a
precedence chain, so a quarantine no longer hides the invalid count.

Adds dur() beside si(): a ms reading picks its own unit, so a slow run reads
"1.24 s" instead of "1.2k ms".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

frontend/src/lib/utils.ts

@@ -41,3 +41,38 @@ export function si(value: number, digits = 3): string {
   const [factor, suffix] = SI_STEPS.find(([step]) => size >= step) ?? [1, ""]
   return `${Number((rounded / factor).toPrecision(digits))}${suffix}`
 }
+
+/** Floor, divisor and unit per step, largest first. */
+const DUR_STEPS: [number, number, string][] = [
+  [60_000, 60_000, "min"],
+  [1_000, 1_000, "s"],
+  [0.01, 1, "ms"],
+  [0, 1e-3, "µs"],
+]
+
+/**
+ * A duration in milliseconds at `digits` significant digits, carrying the unit
+ * it picked: 1240 → "1.24 s", 90000 → "1.5 min", 203.63 → "204 ms".
+ *
+ * Unlike `si` the unit is not the caller's to write. Shortening the number in
+ * front of a fixed " ms" gives "1.2k ms", which is a duration nobody reads —
+ * so the step moves the unit instead, and the value is rounded before that
+ * step is picked, so 999.7 reads "1 s" rather than "1000 ms".
+ *
+ * Below a millisecond it holds down to 0.01, which is `si`'s own plain-number
+ * window transposed onto units. That floor is deliberate: without it a tile
+ * polled every ten seconds flips between "400 µs" and "1.2 ms", and a reading
+ * that changes unit on every poll reads as broken rather than as fast.
+ */
+export function dur(ms: number, digits = 3): string {
+  if (!Number.isFinite(ms)) return "--"
+  const rounded = Number(ms.toPrecision(digits))
+  const size = Math.abs(rounded)
+  if (size === 0) return "0 ms"
+  const [, factor, unit] = DUR_STEPS.find(([floor]) => size >= floor) ?? [
+    0,
+    1,
+    "ms",
+  ]
+  return `${Number((rounded / factor).toPrecision(digits))} ${unit}`
+}