Allow user to switch layouts while phone is in landscape

This fixes a regression on the development branch: the layout switcher would not respond to input while the window mode is 'flat' (i.e. while a mobile phone is in landscape orientation). See https://github.com/element-hq/element-call/pull/3605#discussion_r2586226422 for more context.

I was having a little trouble interpreting the emergent behavior of the layout switching code, so I refactored it in the process into a form that I think is a more direct description of the behavior we want (while not making it as terse as my original implementation).

src/state/CallViewModel/CallViewModel.ts

@@ -946,11 +946,12 @@ export function createCallViewModel$(
     ),
   );
 
-  const hasRemoteScreenShares$: Observable<boolean> = spotlight$.pipe(
-    map((spotlight) =>
-      spotlight.some((vm) => !vm.local && vm instanceof ScreenShareViewModel),
+  const hasRemoteScreenShares$ = scope.behavior<boolean>(
+    spotlight$.pipe(
+      map((spotlight) =>
+        spotlight.some((vm) => !vm.local && vm instanceof ScreenShareViewModel),
+      ),
     ),
-    distinctUntilChanged(),
   );
 
   const pipEnabled$ = scope.behavior(setPipEnabled$, false);