test(web): disable Mantine transition timers in the unit env

CI failed with an unhandled `ReferenceError: window is not defined` thrown from
a Mantine `Transition` `setTimeout` (`use-transition.mjs`) firing after the test
environment was torn down — every test passed, but the leaked timer failed the
run. happy-dom has no `matchMedia`, so `useReducedMotion` resolves to
"motion allowed" and every Transition (ScreenStage, Modal, …) schedules real
enter/exit timers; a timer that outlives its test throws post-teardown. This
surfaced now because MonitoringScreen started wrapping its screens in
ScreenStage (a Transition) where it previously rendered them directly.

Mock `window.matchMedia` in the test setup to report
`prefers-reduced-motion: reduce`, so Mantine transitions render instantly (no
timers). All other queries return `false`, matching the prior absent-matchMedia
behavior, so `useMediaQuery`-driven layout is unchanged. Full unit suite passes
with no unhandled errors across repeated runs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BrihGWcrM9JGRyu41nzZYw

clients/web/src/test/setup.ts

@@ -57,6 +57,32 @@ Object.defineProperty(globalThis, "fetch", {
     ),
 });
 
+// happy-dom doesn't implement `matchMedia`, so Mantine's `useReducedMotion`
+// resolves to "motion allowed" and every `Transition` (ScreenStage, Modal, …)
+// schedules real enter/exit `setTimeout`s. A timer that outlives its test fires
+// after the environment is torn down and throws `window is not defined` from
+// deep in react-dom — an unhandled error that fails the run even though every
+// test passed. Report `prefers-reduced-motion: reduce` so Mantine transitions
+// render instantly (no timers); all other queries stay `false`, matching the
+// prior (absent-matchMedia) behavior so `useMediaQuery`-driven layout is
+// unchanged. Tests that need specific media results still mock `@mantine/hooks`
+// or stub `matchMedia` themselves.
+Object.defineProperty(window, "matchMedia", {
+  configurable: true,
+  writable: true,
+  value: (query: string): MediaQueryList =>
+    ({
+      matches: /prefers-reduced-motion/.test(query),
+      media: query,
+      onchange: null,
+      addEventListener: () => {},
+      removeEventListener: () => {},
+      addListener: () => {},
+      removeListener: () => {},
+      dispatchEvent: () => false,
+    }) as unknown as MediaQueryList,
+});
+
 afterEach(() => {
   cleanup();
   window.localStorage.clear();