Implement most of the remaining layout changes

Includes the mobile UX optimizations and the tweaks we've made to cut down on wasted space, but does not yet include the change to embed the spotlight tile within the grid.

src/room/InCallView.module.css

@@ -68,7 +68,7 @@ limitations under the License.
   align-items: center;
   gap: var(--cpd-space-3x);
   padding-block: var(--cpd-space-4x);
-  margin-inline: var(--inline-content-inset);
+  padding-inline: var(--inline-content-inset);
   background: linear-gradient(
     180deg,
     rgba(0, 0, 0, 0) 0%,
@@ -123,17 +123,16 @@ limitations under the License.
   display: none;
 }
 
+.footer.overlay {
+  position: absolute;
+  inset-block-end: 0;
+  inset-inline: 0;
+}
+
 .fixedGrid {
   position: absolute;
   inline-size: 100%;
   align-self: center;
-  /* Disable pointer events so the overlay doesn't block interaction with
-  elements behind it */
-  pointer-events: none;
-}
-
-.fixedGrid > :not(:first-child) {
-  pointer-events: initial;
 }
 
 .scrollingGrid {
@@ -143,6 +142,18 @@ limitations under the License.
   align-self: center;
 }
 
+.fixedGrid,
+.scrollingGrid {
+  /* Disable pointer events so the overlay doesn't block interaction with
+  elements behind it */
+  pointer-events: none;
+}
+
+.fixedGrid > :not(:first-child),
+.scrollingGrid > :not(:first-child) {
+  pointer-events: initial;
+}
+
 .tile {
   position: absolute;
   inset-block-start: 0;