fix(canvas): derive build elapsed time during render instead of via effect

react-doctor blocked CI with a `no-adjust-state-on-prop-change` error at
CanvasBuildStatus.tsx: the effect called `setNow(Date.now())` to restart the
clock when a new build became active, so the first commit after a build change
briefly rendered the previous build's elapsed time.

The `now` state was only ever read to compute the label, so drop it: the
interval now just bumps a tick to force a re-render and the label is computed
from `Date.now()` during render. This removes the duplicated state (one of the
two remedies react-doctor suggests) rather than adding a `prev`-prop
comparison, and it also means the label can never be stale.

react-doctor and `biome ci` are clean; @posthog/ui typecheck plus the
@posthog/ui (2323) and @posthog/core (2595) unit suites pass.

Generated-By: PostHog Code
Task-Id: 4928140a-2ab8-4fb3-86c1-e02d25f60cca

Author: k11kirky

packages/ui/src/features/canvas/freeform/CanvasBuildStatus.tsx

@@ -70,20 +70,23 @@ export function CanvasBuildStatus({
   );
   const activeId = active?.id;
 
-  // Elapsed-time ticker for the active build. Keyed on the build id so a new
-  // build restarts the clock; idle when no build is active.
-  const [now, setNow] = useState(() => Date.now());
+  // Elapsed-time ticker for the active build: the interval only forces a
+  // re-render, the label is derived from the clock during render. Keyed on the
+  // build id so a new build restarts the ticker; idle when no build is active.
+  const [, setTick] = useState(0);
   useEffect(() => {
     if (!activeId) return;
-    setNow(Date.now());
-    const timer = setInterval(() => setNow(Date.now()), ELAPSED_TICK_MS);
+    const timer = setInterval(
+      () => setTick((tick) => tick + 1),
+      ELAPSED_TICK_MS,
+    );
     return () => clearInterval(timer);
   }, [activeId]);
 
   if (!lifecycle || lifecycle.builds.length === 0) return null;
 
   if (active) {
-    const elapsed = formatElapsed(now - Date.parse(active.createdAt));
+    const elapsed = formatElapsed(Date.now() - Date.parse(active.createdAt));
     return (
       <Flex align="center" gap="1" data-testid="canvas-build-active">
         <SpinnerGapIcon size={14} className="animate-spin text-gray-9" />