feat(web): draw the sidebar's scrollbar only under the pointer

The session list overflows after a handful of sessions, and its scrollbar
was drawn permanently in a column that is at rest most of the time.

SidebarRoot now tracks the pointer over the whole column and rebinds
ui-theme's scrollbar indirection pair to `transparent` while it is
outside, keeping the thumb for 2s after the pointer leaves so it does not
blink out on the way past. Rebinding colour leaves the list's
`scrollbar-gutter: stable` reservation in force, so revealing the bar
moves no row.

ui-theme's gate now states the widened contract: a rebind targets an -l2
token pair or `transparent`, and nothing else.

packages/client/ui-sidebar/tests/__snapshots__/sidebar-snapshot.spec.tsx.snap

@@ -5,7 +5,7 @@ exports[`sidebar shell snapshots > renders the collapsed rail after the crossfad
   data-slot="sidebar"
 >
   <div
-    class="root collapsed railIn"
+    class="root collapsed railIn quietBars"
     style=""
   >
     <div
@@ -65,7 +65,7 @@ exports[`sidebar shell snapshots > renders the expanded column (wordmark, capsul
   data-slot="sidebar"
 >
   <div
-    class="root"
+    class="root quietBars"
     style="width: 300px;"
   >
     <div
@@ -135,7 +135,7 @@ exports[`sidebar shell snapshots > renders the expanded column in the default lo
   data-slot="sidebar"
 >
   <div
-    class="root"
+    class="root quietBars"
     style="width: 300px;"
   >
     <div