fix(ui): floor both halves of a task duration

[ANIRUDDHA ADAK (aniruddhaadak80)]

What

formatTaskDuration in frontend/ui/src/components/research-trace.ts floors the larger unit and rounds the smaller one:

if (value < 3_600_000) return `${Math.floor(value / 60_000)}m ${Math.round((value % 60_000) / 1_000)}s`
return `${Math.floor(value / 3_600_000)}h ${Math.round((value % 3_600_000) / 60_000)}m`

Why it matters

Whenever the remainder is within half a second of the next unit, the rounded value reaches 60 — and nothing carries it, because the larger half was already floored:

formatTaskDuration(119_999)    -> "1m 60s"
formatTaskDuration(3_599_999)   -> "59m 60s"
formatTaskDuration(7_199_999)   -> "1h 60m"

60 is not a valid number of seconds, so a task card shows a duration that does not exist. The window is wide: within [119_500, 3_599,999] there are 29,500 values that print a 60.

The call site (message-part.tsx:1397) passes metadata.durationMs through with no clamp, so any finished Task landing in that window is affected.

The behaviour is pinned by its own sibling — elapsedLabel, six lines above, the counter for a still-ticking task, floors throughout and is right on the same input:

elapsedLabel(119_999)        // "1m 59s"   correct
formatTaskDuration(119_999)  // "1m 60s"   wrong, same value

Verification

New case in the existing duration test. Fails before:

Expected: "1m 59s"
Received: "1m 60s"
(fail) formats compact child durations   <- the new test
 34 pass  1 fail

(pass) formats compact child durations
(pass) never prints a 60 in the smaller unit
 35 pass
 0 fail

The pre-existing assertions (800 / 7,800 / 125,000) are untouched and still pass, so the ordinary values are unchanged. The new test checks all three magnitude branches and asserts the same input through elapsedLabel, which is what makes the floor behaviour a stated contract rather than a coincidence.

The wider frontend/ui suite is flaky in this checkout (231 pass / 12 fail on upstream/main; 232/12 and 233/11 with this change across runs — order and timing dependent), so this is judged on the targeted file.

The change

-  if (value < 3_600_000) return `${Math.floor(value / 60_000)}m ${Math.round((value % 60_000) / 1_000)}s`
-  return `${Math.floor(value / 3_600_000)}h ${Math.round((value % 3_600_000) / 60_000)}m`
+  // Floor both halves. Rounding the remainder reaches 60 and prints a duration
+  // that does not exist, where the ticking counter above floors throughout.
+  if (value < 3_600_000) return `${Math.floor(value / 60_000)}m ${Math.floor((value % 60_000) / 1_000)}s`
+  return `${Math.floor(value / 3_600_000)}h ${Math.floor((value % 3_600_000) / 60_000)}m`

Touched files are Prettier-clean (verified on LF-normalized copies; this Windows checkout's core.autocrlf=true makes Prettier flag every file repo-wide).

Fixes #861

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