feat(keyboard): add Cmd+./Ctrl+. shortcut to toggle both sidebars

Reassign Cmd+. (Mac) and Ctrl+. (Win/Linux) from toggling the left
sidebar to toggling both sidebars at once. If either is open, both
close; if both are closed, both open. Left sidebar toggle on non-Mac
now uses Ctrl+B to match the Mac Cmd+B binding.

packages/app/src/keyboard/actions.ts

@@ -35,6 +35,7 @@ export type KeyboardActionId =
   | "workspace.navigate.relative"
   | "sidebar.toggle.left"
   | "sidebar.toggle.right"
+  | "sidebar.toggle.both"
   | "command-center.toggle"
   | "shortcuts.dialog.toggle"
   | "workspace.terminal.new"