Auto-reveal hidden sidebar on cursor reach

Cursor-reach reveal now fulfils the "Slide in on cursor reach"
description on the design's Hidden-on-hover layout card.

- BrowserCore::active_space_sidebar_width() returns the current
  sidebar tier without cloning a full BrowserSnapshot, so the
  mouse_move hot path stays cheap.
- on_window_mouse_move on the shell root hits-tests the cursor x
  against REVEAL_THRESHOLD_PX (24 px from the left edge). Outside
  the reveal/collapse zones it returns immediately, so 99 % of
  mouse moves never even read the snapshot.
- When in HIDDEN mode and the cursor crosses the reveal threshold,
  expand_hidden_sidebar fires (its early-return on already-expanded
  state prevents notify spam). When the cursor passes
  COLLAPSE_THRESHOLD_PX (shell inset + default sidebar width + 24 px
  buffer), collapse_hidden_sidebar fires.
- Click-to-expand on the rail and click-on-backdrop-to-collapse
  remain as predictable fallbacks.

crates/ely_browser_core/src/state.rs

@@ -465,6 +465,11 @@ impl BrowserCore {
             .ok_or_else(|| CoreError::SpaceNotFound { id: self.active_space_id.clone() })
     }
 
+    #[must_use]
+    pub fn active_space_sidebar_width(&self) -> Option<u16> {
+        self.active_space().ok().map(|space| space.sidebar_width_px())
+    }
+
     fn favorites(&self) -> Vec<BrowserTab> {
         self.tabs.iter().filter(|tab| tab.flags().favorite).cloned().collect()
     }