refactor(app): give formatTime a home — a leaf format.js, one fewer host hook (R3a) (#895)

static/js/format.js (17). One function. Retires the formatTime hook: 12 -> 11.

WHY A MODULE FOR ONE FUNCTION. formatTime was a host hook — loops.js and
section-practice.js both reached back through the seam for it. It is ALSO, by pure
accident of who calls it, inside the dependency closure of the library carve that comes
next. Leaving it there would have made loops.js and section-practice.js import the
LIBRARY in order to format a timestamp — nonsense, and a cycle waiting to happen.

Same rule as the transport carve: a hook is a cycle you agreed to live with; an import is
a dependency you actually have. formatTime has a real owner. It just isn't app.js, and it
certainly isn't the library. Give it a home and both consumers import it directly.

A leaf on purpose. Anything else that turns out to be a shared pure formatter belongs
here too; nothing does yet (I checked — formatBadge, _safeImageUrl and _fetchJsonOrThrow
have no callers outside the library), so nothing else is here.

node 1040/1040, host contract 2/2, ESLint 0.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/app.js

@@ -127,6 +127,7 @@ import {
     toggleSectionPracticePopover,
 } from './js/section-practice.js';
 import { configureHost } from './js/host.js';
+import { formatTime } from './js/format.js';
 // The playback transport. These used to BE app.js — they are imported back now, and the
 // four modules that reached for them through the host seam import them directly instead.
 import {
@@ -4821,7 +4822,6 @@ if (window.feedBack) {
 }
 
 
-function formatTime(s) { return `${Math.floor(s/60)}:${String(Math.floor(s%60)).padStart(2,'0')}`; }
 
 
 
@@ -6261,7 +6261,6 @@ async function checkScanAndLoad() {
 // tests/js/host_contract.test.js fails CI if this list and the host.* uses under
 // static/js/ ever drift apart.
 configureHost({
-    formatTime,
     handleSliderInput,
     playSong,
     // count-in is a module now, so section-practice reaches it through the seam too —