feat(chat): block-level streaming reveal with a gliding follow

Token-by-token streaming mutates the trailing paragraph in place on
every tick: words rewrap, the last line jitters, and the whole reply
reads as flicker. Streamed text now commits only up to the last complete
line — prose arrives a paragraph at a time (a markdown paragraph is one
logical line), code fences reveal line by line, tables row by row — and
a shown block never changes again. A paragraph that runs long without a
newline releases at the last sentence (then word) boundary so the stream
never stalls. Applies to assistant text and reasoning; tool output keeps
its raw tail.

With growth arriving in block steps, the end follow switches to the
list's animated mode so each step is a glide — reveal and scroll read as
one continuous motion. The gesture opt-out now measures at-end from the
live list state instead of the cached flag, which the animated glide
deliberately leaves stale while trailing the edge; without that, a drag
during a glide could leave the scroll-to-bottom pill unshown.

Measured: end-following holds at distance 0 for the whole stream, and
the mobile drag opt-out shows the pill in three of three runs. The
continuous glide costs ~15% more main-thread time per streamed character
than the instant follow — the price of the motion.

packages/ui/src/hooks/useChatTimelineScroll.ts

@@ -7,6 +7,7 @@ import { useUIStore } from '@/stores/useUIStore';
 import {
     CHAT_LIST_ANCHOR_OFFSET,
     getAnchoredTurnMetrics,
+    resolveTimelineIsAtEnd,
     type TimelineListMeasurementState,
     type TimelineScrollMode,
 } from '@/components/chat/lib/scroll/timelineScrollAnchoring';
@@ -204,9 +205,14 @@ export const useChatTimelineScroll = ({
         liveFollowGenerationRef.current = null;
         setUserOwnsScroll(true);
         // The end may already have been left by our own movement, in which
-        // case no further at-end transition will fire. This is an explicit
-        // gesture — show the pill immediately, no debounce.
-        if (!isAtEndRef.current) {
+        // case no further at-end transition will fire — and while an animated
+        // follow glide trails the live edge, isAtEndRef is deliberately not
+        // updated, so measure the real distance instead of trusting it. This
+        // is an explicit gesture — show the pill immediately, no debounce.
+        const listState = listRef.current?.getState();
+        const atEndNow = (listState ? resolveTimelineIsAtEnd(listState) : undefined) ?? isAtEndRef.current;
+        isAtEndRef.current = atEndNow;
+        if (!atEndNow) {
             cancelShowButtonTimer();
             setShowScrollButton(true);
         }