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/locales/en.json

@@ -66,7 +66,7 @@
     "edit": "Edit",
     "undo": "Undo",
     "redo": "Redo",
-    "roomDialog": "Share a live-collaboration session",
+    "roomDialog": "Start live collaboration",
     "createNewRoom": "Create new room"
   },
   "alerts": {