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.check.ts

@@ -1,12 +1,13 @@
 /**
- * The cases `si` gets wrong when its rounding is written the obvious way.
+ * The cases `si` and `dur` get wrong when their rounding is written the
+ * obvious way.
  *
  * Run: `bun src/lib/utils.check.ts` (there is no unit runner; the suite in
  * `tests/` drives a running stack).
  */
 import assert from "node:assert/strict"
 
-import { si } from "./utils"
+import { dur, si } from "./utils"
 
 // Nothing is prefixed in the plain window: these are written with a unit after
 // them, and "400m ms" is not an improvement on "0.4 ms".
@@ -30,3 +31,22 @@ assert.notEqual(si(1234, 4), si(1235, 4))
 assert.equal(si(Number.NaN), "--")
 
 console.log("si: ok")
+
+// The unit follows the size, so no reading is left as a shortened number in
+// front of a unit the call site guessed.
+assert.equal(dur(0), "0 ms")
+assert.equal(dur(203.63), "204 ms")
+assert.equal(dur(1240), "1.24 s")
+assert.equal(dur(90_000), "1.5 min")
+
+// The plain window down to 0.01 ms, so a fast run keeps one unit across polls.
+assert.equal(dur(0.4), "0.4 ms")
+assert.equal(dur(0.0031, 4), "3.1 µs")
+
+// Rounding must not leave a value in the step it just rounded out of.
+assert.equal(dur(999.7), "1 s")
+assert.equal(dur(59_999), "1 min")
+
+assert.equal(dur(Number.NaN), "--")
+
+console.log("dur: ok")