fix: scope the "Show in folder" reveal scroll to the tree body

A reveal on a deep tree shifted the whole panel (tab bar included) out
of the viewport: the reveal used native scrollIntoView, which scrolls
every scrollable ancestor, and the fixed panel host
([data-dsh-panel-host], overflow hidden) is still a programmatically
scrollable container — the host scrolled instead of (or besides) the
tree, with no way to scroll back. Reported on DSH 0.1.2-alpha.1.

Two layers:
- FileTree: compute the centered, clamped target and scrollTo the tree
  body only; scrollIntoView is never called.
- panel host: overflow hidden -> clip. A hidden box remains
  programmatically scrollable; clip keeps the same viewport-edge
  clipping (collapsed panels sliding off-screen) while making the host
  impossible to scroll, killing the whole leak class (md-toc jumps,
  future consumer scrollIntoView calls).

src/client/sidebar.module.css

@@ -44,8 +44,12 @@
      page draggable left-right and up-down while any panel was collapsed.
      The host is exactly viewport-sized (inset: 0), so the clip is invisible
      until a panel slides in from the edge; internal scroll containers and
-     the app's fixed overlay stack are unaffected. */
-  overflow: hidden;
+     the app's fixed overlay stack are unaffected. `clip`, not `hidden`: a
+     hidden box stays a PROGRAMMATICALLY scrollable container, and a
+     scrollIntoView fired from panel content (the file tree's "Show in
+     folder" reveal) scrolled the host itself — shifting the whole panel,
+     tab bar included, out of the viewport with no way to scroll back. */
+  overflow: clip;
 }
 
 :global([data-dsh-panel-host][data-dsh-panel-host-degraded]) {