Add hidden-on-hover sidebar mode

Domain ships HIDDEN_SIDEBAR_WIDTH_PX = 8 alongside the existing
collapsed/default tiers. ElyShell tracks sidebar_hover_expanded with
expand_hidden_sidebar / collapse_hidden_sidebar helpers; switching
back to a non-hidden width via the layout cards or core API resets
the flag automatically so the sidebar can never be both hidden and
expanded after a mode change.

Renderer:
- render_sidebar takes a sidebar_hidden flag and routes to a thin
  8 px clickable rail (hover bg + click expands) when the active
  space's width is at HIDDEN.
- While the rail is expanded, render_browser overlays a transparent
  backdrop + the full default-width sidebar absolutely positioned in
  the shell inset, so the main pane content never reflows.
- collapsed_sidebar_active still drives the COLLAPSED-tier compact
  sidebar; the hidden tier is opted out of that path.

Appearance form:
- Layout cards section gains the design's third "Hidden on hover"
  card with a 6 px sliver preview that mutates the active space to
  HIDDEN_SIDEBAR_WIDTH_PX. LayoutMode now derives id/width/preview
  from a small enum so adding a fourth mode would be one match arm.

The hover-expanded state never persists into the domain; once the
user switches modes or clicks the backdrop, it collapses cleanly.

crates/ely_app/src/shell/sidebar.rs

@@ -1,7 +1,8 @@
 use ely_browser_core::BrowserSnapshot;
 use ely_design_system::{colors, spacing};
 use ely_domain::{
-    ArchivedTab, BrowserTab, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, Space,
+    ArchivedTab, BrowserTab, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX,
+    HIDDEN_SIDEBAR_WIDTH_PX, Space,
 };
 use gpui::{
     AnyElement, BoxShadow, Context, IntoElement, ParentElement, Styled, Window, div, hsla, point,
@@ -124,6 +125,23 @@ impl ElyShell {
         };
 
         if core.set_space_sidebar_width(&active_space_id, width_px).is_ok() {
+            if width_px > HIDDEN_SIDEBAR_WIDTH_PX {
+                self.sidebar_hover_expanded = false;
+            }
+            cx.notify();
+        }
+    }
+
+    pub(crate) fn expand_hidden_sidebar(&mut self, cx: &mut Context<Self>) {
+        if !self.sidebar_hover_expanded {
+            self.sidebar_hover_expanded = true;
+            cx.notify();
+        }
+    }
+
+    pub(crate) fn collapse_hidden_sidebar(&mut self, cx: &mut Context<Self>) {
+        if self.sidebar_hover_expanded {
+            self.sidebar_hover_expanded = false;
             cx.notify();
         }
     }