docs(desktop): name the timer drain in the effect that performs it

Review follow-up, comments and one type annotation. No behaviour change.

The unmount effect now clears pending timers, but its leading comment is
still entirely about the focus bus, so the cleanup reads as unrelated code
that happens to be in the same block. Say why it lives there: both concerns
are "this composer is going away", they unmount together by definition, and
a sibling unmount-only effect would only be a second place to forget.

In the regression test, the clearTimeout mock declared id as number. Nothing
that reaches it is a number: jsdom under node returns a Timeout object, which
is why the scheduled and cleared arrays are unknown[] and compared by
identity. The annotation documented a shape the test never sees, so it is now
unknown with the cast moved to the one call that genuinely wants a number.

apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx

@@ -140,6 +140,11 @@ export const UserEditComposer: FC<UserEditComposerProps> = ({ cwd, gateway, sess
   // bus leaks: confirming or cancelling an edit tears the composer down while
   // `'edit'` is still the active target. Release it alongside the thread-scroll
   // cleanup so keyboard routing falls back to the visible chat composer.
+  //
+  // It also drains whatever `scheduleTimeout` still has pending, which is a
+  // second concern under the same heading rather than a separate one: both
+  // are "this composer is going away", they unmount together by definition,
+  // and a sibling unmount-only effect would only be a second place to forget.
   useEffect(
     () => () => {
       notifyThreadEditClose()

apps/desktop/src/components/assistant-ui/thread/user-message-edit.test.tsx

@@ -263,9 +263,13 @@ describe('Enter submission and latch behavior', () => {
       return id
     }) as typeof window.setTimeout)
 
-    vi.spyOn(window, 'clearTimeout').mockImplementation(((id?: number) => {
+    // `id` is typed `unknown` for the same reason the arrays above are: what
+    // actually arrives is whatever `setTimeout` returned, and under jsdom that
+    // is a Timeout object rather than the `number` the DOM lib promises.
+    // Declaring it `number` would have documented a shape this never sees.
+    vi.spyOn(window, 'clearTimeout').mockImplementation(((id?: unknown) => {
       cleared.push(id)
-      realClearTimeout(id)
+      realClearTimeout(id as number | undefined)
     }) as typeof window.clearTimeout)
 
     const onEdit = vi.fn(async () => {})