Restyle the mobile UI a bit (#1002)

* Restyle the bottom bar on mobile as an Island

* Shorter label for collaboration button, truncate too-long button labels

* Refactor safe area things to global vars

* Fix scroll bar positioning, don’t block scrollbars with menu island

* Update text

src/utils.ts

@@ -194,3 +194,9 @@ export function sceneCoordsToViewportCoords(
 
   return { x, y };
 }
+
+export function getGlobalCSSVariable(name: string) {
+  return getComputedStyle(document.documentElement).getPropertyValue(
+    `--${name}`,
+  );
+}