feat(chat): status row and scroll pill share one anchored slot

The streaming status and the scroll-to-bottom pill now hand off to each
other in one place above the composer: same anchor, same input-aligned
column, both in the popover glass (ring and soft shadow stay pill-only
as the interactivity cue, with the shadow on a wrapper so the glass
backdrop-filter cannot drop it after hide/show cycles). The pill is a
single full-surface button, moves to the left edge, and while the
session streams it carries a compact one-line status label instead of
the status row's animation machinery, which did not survive a 32px
chip.

Behavioral fixes along the way: a gesture shows the pill immediately
(no debounce) and hides the status in the same frame; gestures register
whenever the viewport can actually move up — an anchored short turn
kept live-follow armed and suppressed the pill entirely; the status
row sheds its in-list morph offsets (mb-6, message column) and its
inline-size container gets the glass on an inner row, since a query
container cannot shrink-wrap; its working text uses full
muted-foreground to match the pill.

packages/ui/src/components/chat/message/parts/WorkingPlaceholder.tsx

@@ -229,12 +229,11 @@ export function WorkingPlaceholder({
 
   return (
     <div
-      // Styled to mirror the turn footer's model row (text-sm,
-      // muted-foreground/60, no left inset): when the turn completes this row
-      // disappears and the footer appears in the same visual spot, so the two
-      // must read as the same line swapping its text.
+      // Full muted-foreground, matching the scroll-to-bottom pill's status
+      // text: the row and the pill hand off to each other in the same spot
+      // and must read as one element changing chrome.
       className={
-        'flex h-full items-center text-muted-foreground/60'
+        'flex h-full items-center text-muted-foreground'
       }
       role="status"
       aria-live={displayedPermission ? 'assertive' : 'polite'}