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/mod.rs

@@ -66,6 +66,7 @@ pub struct ElyShell {
     pub(crate) plugin_search_input: Entity<InputState>,
     pub(crate) translucency_slider: Entity<SliderState>,
     pub(crate) workspace_picker_open: bool,
+    pub(crate) sidebar_hover_expanded: bool,
     download_action_error: Option<String>,
     download_clear_confirmation: bool,
     download_security_confirmation: Option<PendingDownloadFileAction>,
@@ -177,6 +178,7 @@ impl ElyShell {
             plugin_search_input,
             translucency_slider,
             workspace_picker_open: false,
+            sidebar_hover_expanded: false,
             download_action_error: None,
             download_clear_confirmation: false,
             download_security_confirmation: None,