fix(test): cancel Navbar theme-transition timer on unmount

The dark-mode toggle kicked off a 360ms setTimeout that removed a
CSS class via 'document.documentElement'. In vitest the document
was torn down before the timer fired, triggering an unhandled
ReferenceError that flipped the whole run to a non-zero exit even
though every test passed.

Track the handle in a ref and clearTimeout on unmount (and before
scheduling a new one).

client/src/components/Layout/Navbar.tsx

@@ -61,11 +61,25 @@ export default function Navbar({ tripTitle, tripId, onBack, showBack, onShare }:
     navigate('/login', { state: { noRedirect: true } })
   }
 
+  // Keep track of the pending theme-transition cleanup so we can cancel it
+  // on unmount. Without this the timer fires after jsdom teardown in unit
+  // tests (document is gone) and triggers an unhandled ReferenceError that
+  // trips vitest's exit code.
+  const themeTransitionTimer = useRef<number | null>(null)
+  useEffect(() => () => {
+    if (themeTransitionTimer.current !== null) {
+      window.clearTimeout(themeTransitionTimer.current)
+      themeTransitionTimer.current = null
+    }
+  }, [])
+
   const toggleDarkMode = () => {
     document.documentElement.classList.add('trek-theme-transitioning')
     updateSetting('dark_mode', dark ? 'light' : 'dark').catch(() => {})
-    window.setTimeout(() => {
+    if (themeTransitionTimer.current !== null) window.clearTimeout(themeTransitionTimer.current)
+    themeTransitionTimer.current = window.setTimeout(() => {
       document.documentElement.classList.remove('trek-theme-transitioning')
+      themeTransitionTimer.current = null
     }, 360)
   }