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_domain/src/lib.rs

@@ -58,7 +58,10 @@ pub use site_permission::{
     SiteOrigin, SitePermissionAuditAction, SitePermissionAuditEvent, SitePermissionDecision,
     SitePermissionEntry, SitePermissionFeature,
 };
-pub use space::{ArchivePolicy, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, Space};
+pub use space::{
+    ArchivePolicy, COLLAPSED_SIDEBAR_WIDTH_PX, DEFAULT_SIDEBAR_WIDTH_PX, HIDDEN_SIDEBAR_WIDTH_PX,
+    Space,
+};
 pub use split::{MAX_SPLIT_PANES, SplitAxis, SplitLayout, SplitPane};
 pub use sync::{
     SyncConnectionState, SyncObjectKind, SyncObjectPolicy, SyncObjectState, SyncObjectStatus,