fix(setup): use fmtDuration in the container-build spinner

setup/lib/windowed-runner.ts was the one place on main still printing
elapsed time as raw seconds (`(170s)`) instead of using the
minute-aware `fmtDuration` helper from #2108. Two spots — the live
spinner suffix that ticks during the build, and the
success/error completion suffix — both now go through `fmtDuration`,
so anything past 60 seconds renders as `Xm Ys` (e.g. `2m 50s`) like
the rest of the setup flow.

The miss happened because a separate PR (closed) was supposed to
remove the timer entirely from this file, so #2108 deliberately
skipped it. With that other PR closed, applying `fmtDuration` here
is the consistent fix.

Pure formatting change. The helper itself is unchanged from #2108;
behavior under 60s is identical (`Xs`); behavior past 60s now
matches everywhere else.

setup/lib/windowed-runner.ts

@@ -23,7 +23,7 @@ import { emit as phEmit } from './diagnostics.js';
 import type { StepResult, SpinnerLabels } from './runner.js';
 import { dumpTranscriptOnFailure, spawnStep, writeStepEntry } from './runner.js';
 import * as setupLog from '../logs.js';
-import { brandBody, fitToWidth } from './theme.js';
+import { brandBody, fitToWidth, fmtDuration } from './theme.js';
 
 const WINDOW_SIZE = 3;
 const SPINNER_FRAMES = ['◒', '◐', '◓', '◑'];
@@ -85,9 +85,8 @@ async function runUnderWindow(
   const redraw = (): void => {
     if (stallPromptActive) return;
     out.write(`\x1b[${WINDOW_SIZE + 1}A`);
-    const elapsed = Math.round((Date.now() - start) / 1000);
     const icon = SPINNER_FRAMES[frameIdx % SPINNER_FRAMES.length];
-    const suffix = ` (${elapsed}s)`;
+    const suffix = ` (${fmtDuration(Date.now() - start)})`;
     const header = fitToWidth(labels.running, suffix);
     out.write(`\x1b[2K${k.cyan(icon)}  ${header}${k.dim(suffix)}\n`);
 
@@ -164,8 +163,7 @@ async function runUnderWindow(
   out.write(SHOW_CURSOR);
   process.off('exit', restoreCursorOnExit);
 
-  const elapsed = Math.round((Date.now() - start) / 1000);
-  const suffix = ` (${elapsed}s)`;
+  const suffix = ` (${fmtDuration(Date.now() - start)})`;
   if (result.ok) {
     const isSkipped = result.terminal?.fields.STATUS === 'skipped';
     const msg = isSkipped && labels.skipped ? labels.skipped : labels.done;