chrome: collapsible side panel

This implements the initial logic that makes the side panel collapsible.

Since we don't have a proper hamburger control, we do the same thing we
do on iOS for now.

crates/notedeck/src/app.rs

@@ -19,6 +19,7 @@ use tracing::{error, info};
 
 pub enum AppAction {
     Note(NoteAction),
+    ToggleChrome,
 }
 
 pub trait App {