Fix chat auto-scroll on touch devices (iOS) (#2390)

* fix(chat): use instant scroll on touch to land the spacer pin on iOS

On send, the chat inflates a tall bottom spacer and scrolls to the bottom so
the just-sent user message pins near the top (ChatGPT-style). That scroll was
issued with behavior: "smooth". On iOS, WebKit suppresses/defers programmatic
scrolls during touch/momentum, and the deferred smooth animation only plays
out once the gesture and stream settle — i.e. the view visibly scrolls up
right as the reply finishes.

Use an instant scroll on touch devices (pointer: coarse) so the spacer that
was added is snapped into view immediately on send, with no late animation.
Desktop keeps its smooth pin. The mobile pin behaviour itself is preserved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZA6zBsc61PSuo8kg4yRB7

* fix(chat): use any-pointer:coarse so hybrid touch devices snap on send

pointer:coarse only reflects the primary pointer, so a hybrid device (touch
laptop, iPad with a trackpad) reports fine and keeps the smooth scroll that
WebKit defers when touch-scrolling. Switch to any-pointer:coarse so any
touch-capable device snaps to the bottom immediately on send.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZA6zBsc61PSuo8kg4yRB7

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: gary149

src/lib/components/chat/ChatWindow.svelte

@@ -337,7 +337,13 @@
 				last?.content === "";
 
 			if (userJustSentMessage) {
-				scrollBehavior = "smooth";
+				// Touch devices (iOS): a smooth programmatic scroll is suppressed during
+				// touch/momentum and only animates once the gesture/stream settle — i.e. the
+				// view visibly scrolls up right as the reply finishes. An instant scroll snaps
+				// to the bottom (revealing the freshly inflated spacer) immediately on send.
+				// `any-pointer: coarse` so hybrid devices (touch laptop, iPad + trackpad) that
+				// can still touch-scroll snap instantly even when the primary pointer is fine.
+				scrollBehavior = window.matchMedia("(any-pointer: coarse)").matches ? "instant" : "smooth";
 				forceReattach++;
 				// Only activate dynamic spacer after the first exchange
 				// (first user+assistant pair scrolls normally)