Add workspace switcher disclosure list

Clicking the picker pill now opens an in-flow space-list disclosure
beneath the workspace row instead of cycling to the next space. The
list shows every space's emoji glyph + name, highlights the active one
with the accent check, and on row click switches to that space and
closes the disclosure.

Sidebar disclosure rules:
- The chevron flips ChevronDown → ChevronUp when open so the affordance
  reads at a glance.
- A "Manage spaces" footer routes to ely://settings/spaces and closes
  the picker.
- ElyShell carries a workspace_picker_open: bool with toggle / close /
  select_space_from_picker helpers; render_sidebar_header receives a
  &ElyShell so it can read the open flag without leaking shell internals
  to free helpers.

The existing SelectNextSpace shortcut still routes through
cycle_to_next_space, so the keyboard cycle behaviour is unchanged.

crates/ely_app/src/shell/chrome/sidebar.rs

@@ -28,7 +28,7 @@ impl ElyShell {
             .bg(rgba(panel_color))
             .shadow(panel_shadow())
             .overflow_hidden()
-            .child(render_sidebar_header(snapshot, cx))
+            .child(render_sidebar_header(self, snapshot, cx))
             .child(
                 div()
                     .flex_1()