fix(desktop): restore overflow-anchor:none on live scroll containers

useAnchoredScroll is meant to be the single owner of scrollTop, but both
live scroll containers had lost the [overflow-anchor:none] class (it
survived only on the thread loading skeleton, which never scrolls). With
it gone, Chromium's native scroll-anchoring heuristic re-engaged and
applied its own scrollTop correction on prepend — picking its own anchor
element — while the hook applied a second scrollBy correction on top. When
the two anchors diverged the corrections stacked, producing the residual
jiggle that survived the single-owner rewrite.

Restoring the class on both live containers (MessageTimeline timeline +
MessageThreadPanel body) hands scroll ownership back to the hook alone.

This is a real-wheel-only symptom: native scroll anchoring barely fires
under synthetic scrollTop= writes, so the headless suite stayed green
while a manual macOS scroll pass surfaced it. scroll-history e2e 6/6,
tsc clean.

Co-authored-by: Tyler Longwell <tlongwell@squareup.com>
Signed-off-by: Tyler Longwell <tlongwell@squareup.com>

desktop/src/features/messages/ui/MessageThreadPanel.tsx

@@ -383,7 +383,7 @@ export function MessageThreadPanel({
   const threadScrollRegion = (
     <div
       className={cn(
-        "min-h-0 flex-1 overflow-y-auto overflow-x-hidden overscroll-contain pb-24",
+        "min-h-0 flex-1 overflow-y-auto overflow-x-hidden overscroll-contain pb-24 [overflow-anchor:none]",
         isSplitLayout && auxiliaryPanelContentPaddingClass,
         !isSplitLayout && !isFloatingOverlay && "pt-[4.75rem]",
       )}

desktop/src/features/messages/ui/MessageTimeline.tsx

@@ -307,7 +307,7 @@ const MessageTimelineBase = React.forwardRef<
         ) : null}
         <div
           className={cn(
-            "absolute inset-0 overflow-y-auto overflow-x-hidden overscroll-contain px-2 pt-1",
+            "absolute inset-0 overflow-y-auto overflow-x-hidden overscroll-contain px-2 pt-1 [overflow-anchor:none]",
             hasComposerOverlay ? "pb-24" : "pb-4",
           )}
           data-scroll-restoration-id={scrollRestorationId}