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/LayoutSwitch.test.ts

@@ -107,3 +107,26 @@ test("auto-switches to spotlight when in flat window mode", () =>
     windowMode: "      nNf",
     expectedGridMode: "g-s",
   }));
+
+test("allows switching modes manually when in flat window mode", () =>
+  testLayoutSwitch({
+    // Window becomes flat, then user switches to grid and back.
+    // Finally the window returns to a normal shape.
+    windowMode: "      nf--n",
+    userSelection: "   --gs",
+    expectedGridMode: "gsgsg",
+  }));
+
+test("stays in spotlight while there are screen shares even when window mode changes", () =>
+  testLayoutSwitch({
+    windowMode: "      nfn",
+    hasScreenShares: " y",
+    expectedGridMode: "s",
+  }));
+
+test("ignores end of screen share until window mode returns to normal", () =>
+  testLayoutSwitch({
+    windowMode: "      nf-n",
+    hasScreenShares: " y-n",
+    expectedGridMode: "s--g",
+  }));