test(desktop): unmount every TerminalSubstrate tree before the window closes (#9440)

## Summary
`desktop/src/features/terminal/TerminalSubstrate.test.mjs` can pass
every test and then run the Desktop JS job out of memory several minutes
later. This moves the React Testing Library unmount from `beforeEach` to
`afterEach`, so no tree is still mounted when the jsdom window closes.
Only the test file changes (6 insertions, 2 deletions).

Commit (cherry-picked unchanged from a local flake-fix series):
- `test(desktop): unmount every TerminalSubstrate tree before the window
closes`

### CI sighting

One sighting was found in the CI runs from 7 to 12 November 2026 that
were searched:

- 9 November 2026, "Desktop Domain / Desktop JS", run 42706004493
attempt 1 (on #9227; the rerun passed):
-
https://github.com/block/buzz/actions/runs/42706004493/job/129463488242

In that job the suite reported 6,950 of 6,951 passing. The log shows `An
update to TerminalSubstrate inside a test was not wrapped in act(...)`
at 21:54:33Z, then `FATAL ERROR: Ineffective mark-compacts near heap
limit Allocation failed - JavaScript heap out of memory` at 22:02:17Z,
and the file is reported as failed at file level (`test at
src/features/terminal/TerminalSubstrate.test.mjs:1:1`) after 504.6 s.

### Root cause

The file called `cleanup()` only in `beforeEach`, so the last test's
tree was still mounted when `after(() => dom.window.close())` ran. If
ThemeProvider's asynchronous palette update landed after that close, it
re-rendered the substrate, the splash effect's palette guard passed, and
`requestAnimationFrame` on the closed window started a fresh jsdom
interval that nothing cleared. That 60 Hz loop repainted the banner into
the test's recording `getContext` stub indefinitely, and `paintLog` grew
until the heap limit. No individual test hangs, so a per-test timeout
would not catch it.

### Fix

`afterEach(() => cleanup?.())` unmounts every tree, including the last,
before `after()` closes the window. The `beforeEach` call became
redundant and is removed. No assertions change, and no timeouts, retries
or sleeps are added.

### Related issue
N/A (flake fix). No existing issue or PR for this failure was found.
Three open PRs touch the same test file (#5057, #5485, #7777); none of
them changes where `cleanup` runs.

### Testing
Human testing is still required. This PR does not carry
`buzz-review-completed`; the author has not yet run this test themselves
on this branch.

#### Verification of the fix (before this branch was split out)
- The test file, 5 consecutive runs: 25 of 25 tests passed each time in
about 6.8 s, with no `act` warning.
- The CI out-of-memory failure was not reproduced with the real test
file. Its mechanism was reproduced in a scratch harness with the same
stubs: without cleanup before the window close, 3 of 3 runs left the
palette unresolved at close, added 11,556 `paintLog` entries per 100 ms,
kept one live timer, and never exited. With cleanup before the close, 3
of 3 runs added no entries, left no timers, and exited on their own.

#### Local checks on this branch
The push ran the repository's full pre-push hook set from a checkout of
this branch's head (`7f4117340`), with no lanes excluded and without
`--no-verify`. The branch is one commit behind `main`; that commit
changes only mobile and docs files, none of them touched here.

Ran and passed:
- `branch-skew`, `push-head-scope` and `file-size-check`
- `desktop-check` (`pnpm check`)
- `desktop-typecheck` (`pnpm typecheck`)
- `desktop-test` (`pnpm test`, the full desktop unit suite, which
includes `TerminalSubstrate.test.mjs`)

Did not run:
- `rust-tests`, `desktop-tauri-checks` and `mobile-checks`. The hook
selects lanes by changed path, and this branch changes no Rust, Tauri or
mobile file.
- Desktop builds and the Playwright E2E suites, which are CI-only.
- `just ci`.
- The 5-run repetition and the scratch harness above were not repeated
on this branch.

A green run does not prove the flake is gone: the failure needs the
palette update to land after the window closes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: mtoohey <mtoohey@squareup.com>
Signed-off-by: Matt Toohey <contact@matttoohey.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

desktop/src/features/terminal/TerminalSubstrate.test.mjs

@@ -1,5 +1,5 @@
 import assert from "node:assert/strict";
-import { after, before, beforeEach, test } from "node:test";
+import { after, afterEach, before, beforeEach, test } from "node:test";
 
 import { JSDOM } from "jsdom";
 
@@ -67,8 +67,12 @@ before(async () => {
 });
 
 after(() => dom.window.close());
+// Unmount after every test, not before the next one: a tree still mounted when
+// the window closes can be re-rendered by ThemeProvider's late palette update,
+// whose `requestAnimationFrame` on the closed window starts a jsdom interval
+// that nothing clears and that fills `paintLog` until the heap limit.
+afterEach(() => cleanup?.());
 beforeEach(() => {
-  cleanup?.();
   reducedMotion = false;
   paintLog.length = 0;
   dom.window.localStorage.clear();