fix(markdown): preview TOC dismiss-on-outside-click, popover z-index, code-block banner stickiness

Three preview (render) bugs where DSH's chat-optimized markdown primitives
leak into the file-preview scroll world:

- CodeBlock banner: DSH's bannerWrap is position:sticky (top:0, z-index:6) so a
  block keeps its copy action reachable inside a per-message scroll container.
  In the preview every fence shares one .editorMd scroll container, so the
  banner pins to the viewport top until the whole block scrolls past and reads
  as a floating header. Neutralize stickiness for non-mermaid fences in the
  preview (mermaid chrome replaces the banner and is excluded via
  data-mermaid-processed).

- TOC popover z-index: .tocBar was z-index:3, below the code banner's 6, so the
  outline panel painted beneath a sticky fence header. Raise the bar to 7.

- TOC dismissal: only the toggle re-click closed the popover. Add a document
  pointerdown listener that closes on an outside press (blank preview space),
  exempting the button and panel so the toggle still flips.

Adds md-toc unit coverage for outside-click dismiss and toggle-preserving
behavior; records the deviations in the TOC design doc.

src/client/md-toc.tsx

@@ -68,13 +68,30 @@ export function MdToc(): ReactNode {
 
   // Esc closes the popover (the panel is a sibling of the button, so focus
   // stays wherever the user came from — a document listener is the simplest).
+  // A document pointerdown also closes it when the click lands outside the
+  // button and the panel, so clicking blank preview space dismisses the
+  // outline instead of requiring a second click on the toggle. The check runs
+  // at pointerdown (before the button's own click toggles), and the button and
+  // panel are exempt so a re-click on the toggle keeps working as a toggle
+  // rather than being eaten by the dismiss.
   useLayoutEffect(() => {
     if (!open) return
     const onKey = (event: KeyboardEvent): void => {
       if (event.key === 'Escape') setOpen(false)
     }
+    const onPointerDown = (event: Event): void => {
+      const target = event.target
+      if (!(target instanceof Node)) return
+      const bar = barRef.current
+      if (bar !== null && bar.contains(target)) return
+      setOpen(false)
+    }
     document.addEventListener('keydown', onKey)
-    return () => { document.removeEventListener('keydown', onKey) }
+    document.addEventListener('pointerdown', onPointerDown)
+    return () => {
+      document.removeEventListener('keydown', onKey)
+      document.removeEventListener('pointerdown', onPointerDown)
+    }
   }, [open])
 
   const jump = (entry: TocEntry): void => {