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/FileTree.tsx

@@ -280,10 +280,21 @@ export function FileTree(props: {
   // (revealPaths), but the row may not be scrolled into sight — a reveal on
   // a long tree should surface the highlighted file. Re-runs when the tree
   // data or reveal set changes (the row appears after its level loads).
+  // Scrolls ONLY this tree's body: scrollIntoView would scroll every
+  // scrollable ancestor, and the clipping panel host
+  // ([data-dsh-panel-host]) is still programmatically scrollable — a deep
+  // reveal shifted the whole panel, tab bar included, out of the viewport.
   useEffect(() => {
     if (revealed.length === 0) return
-    const row = bodyRef.current?.querySelector('[data-dsh-revealed]')
-    row?.scrollIntoView({ block: 'center', behavior: 'smooth' })
+    const body = bodyRef.current
+    if (body === null) return
+    const row = body.querySelector<HTMLElement>('[data-dsh-revealed]')
+    if (row === null) return
+    const bodyTop = body.getBoundingClientRect().top
+    const rowRect = row.getBoundingClientRect()
+    const target = body.scrollTop + (rowRect.top + rowRect.height / 2) - (bodyTop + body.clientHeight / 2)
+    const max = Math.max(body.scrollHeight - body.clientHeight, 0)
+    body.scrollTo({ top: Math.min(Math.max(target, 0), max), behavior: 'smooth' })
   }, [revealed, data])
 
   /** Copy `text`; on success flip the row's copied label for a moment. */

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]) {