fix(chat): use useLayoutEffect for scroll restore + auto-bottom to avoid mount flicker

The scroll-position memory introduced in 0.17.2 still produced a visible
"chat appears at the top then jumps" frame when switching back into a
conversation. Cause: both scroll-affecting effects (auto-bottom on new
messages, restore on chat re-entry) used useEffect, which fires AFTER
the browser paints the freshly-committed DOM. So users saw scrollTop=0
for one frame before the effect ran and corrected it.

Switching both to useLayoutEffect moves the scroll write into the same
commit phase as the message-list DOM update, so the very first paint
already shows the correct position — single paint, no flicker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src/pages/ConversationPage.tsx

@@ -1,6 +1,6 @@
 import { parseMessagePayload } from '@chat-app/shared/chat';
 import { extractErrorCode } from '@chat-app/shared/i18n';
-import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
+import { useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState } from 'react';
 import { useTranslation } from 'react-i18next';
 import { useParams } from 'react-router-dom';
 
@@ -406,7 +406,14 @@ export function ConversationPage() {
     if (id && messages.length > 0) markRead(id);
   }, [id, messages.length, markRead]);
 
-  useEffect(() => {
+  // useLayoutEffect: run synchronously after DOM commit, before the
+  // browser paints. Using useEffect here let one frame of "scrollTop = 0
+  // (top of list)" paint between message-list mount and the auto-scroll,
+  // which is exactly the "flickers to a different position, then jumps"
+  // glitch users saw when re-entering a chat. Layout-effect fires while
+  // the message list is in the DOM but before paint, so the first frame
+  // already shows the correct scroll position.
+  useLayoutEffect(() => {
     const el = scrollRef.current;
     if (!el || !stickToBottom) return;
     el.scrollTop = el.scrollHeight;
@@ -423,7 +430,13 @@ export function ConversationPage() {
   const restoredForRef = useRef<string | null>(null);
   const isRestoringRef = useRef(false);
 
-  useEffect(() => {
+  // useLayoutEffect, same reason as above: writing scrollTop here happens
+  // before the first paint of the freshly-mounted chat, so the user
+  // doesn't see a frame at scrollTop=0 before the jump to the saved
+  // position. Combined with the messages.length gate this means the
+  // re-entry shows the message list AT the saved scroll location in one
+  // single paint — no "loaded then jumped" effect.
+  useLayoutEffect(() => {
     const el = scrollRef.current;
     if (!el || !id) return;
     if (restoredForRef.current === id) return;