fix(chat): anchored-scroll behavior parity with the reference model

Six reported defects, one root theme — our port diverged from the
reference semantics:

- a user gesture no longer collapses the reserved anchored end space
  (that snap-to-bottom was the 'anchoring vanishes on a micro-scroll');
  it only disarms the follow/anchor machinery, and anchor remeasures
  after the user takes over can no longer re-position the viewport
- the anchor arms synchronously BEFORE the send reaches the store, and
  the claim compares against the arm-time baseline — arming a frame
  after the optimistic row committed is why anchoring 'rarely worked'
  and sending from mid-history did nothing
- gestures are selective (wheel up, touch/pointer away from the end,
  PageUp/Home/ArrowUp), so scrolling toward the end or clicking a row
  at the live edge no longer kills follow
- 'at end' is a tight 40px band against the full content length instead
  of the list's half-viewport isNearEnd, so the scroll-to-bottom pill
  appears when the viewport actually leaves the end
- reaching the end re-arms follow without knocking an anchored turn out
  of its mode, and the overlay scrollbar suppression follows scroll
  ownership rather than the anchor's mere existence
- the status/working row moved out of the list footer to a fixed spot
  above the composer: inside the list it walked down with every
  streamed line, and its unmount was part of the end-of-stream jerk

packages/ui/src/components/chat/ChatContainer.tsx

@@ -361,10 +361,6 @@ const ChatViewport = React.memo(({
 
             <SessionRecapNote sessionId={currentSessionId} directory={directory} isMobile={isMobile} />
 
-            <div className="mb-3">
-                <StatusRowContainer />
-            </div>
-
             <div className="flex-shrink-0" style={{ height: isMobile ? '40px' : '10vh' }} aria-hidden="true" />
         </>
     ), [currentSessionId, directory, isMobile, sessionPermissions, sessionQuestions]);
@@ -417,6 +413,13 @@ const ChatViewport = React.memo(({
                     scrollContainerProps={scrollContainerProps}
                 />
                 <OverlayScrollbar containerRef={scrollRef} suppressVisibility={isProgrammaticFollowActive} userIntentOnly observeMutations={false} />
+                {/* Static above the composer: inside the list it walked down
+                    with every streamed line while a turn was anchored. */}
+                <div className="pointer-events-none absolute inset-x-0 bottom-2 z-10">
+                    <div className="pointer-events-auto [&:not(:has(*))]:hidden">
+                        <StatusRowContainer />
+                    </div>
+                </div>
                 {showPromptNavigator && promptTurnIds.length >= 2 ? (
                     <PromptNavigatorRail
                         turnIds={promptTurnIds}