feat(chat): add scroll position memory per conversation (M8.10)

Save scroll position when switching conversations, restore it when
returning. Uses a session-only Map<conversationId, scrollTop>.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/components/chat/ChatWindow.vue

@@ -137,6 +137,27 @@ const codeContext = useCodeContext()
 const messageListRef = ref<HTMLElement | null>(null)
 const chatSearchRef = ref<InstanceType<typeof ChatSearch> | null>(null)
 
+// Scroll position memory per conversation
+const scrollPositions = new Map<string, number>()
+
+function saveScrollPosition() {
+  const el = messageListRef.value
+  const id = chatStore.activeConversationId
+  if (el && id) {
+    scrollPositions.set(id, el.scrollTop)
+  }
+}
+
+function restoreScrollPosition(convId: string) {
+  const saved = scrollPositions.get(convId)
+  if (saved !== undefined) {
+    nextTick(() => {
+      const el = messageListRef.value
+      if (el) el.scrollTop = saved
+    })
+  }
+}
+
 function scrollToMessageIndex(index: number) {
   virtualizer.value.scrollToIndex(index, { align: 'center' })
 }
@@ -313,6 +334,15 @@ function scrollToBottom() {
   })
 }
 
+// Save/restore scroll position on conversation switch
+watch(
+  () => chatStore.activeConversationId,
+  (newId, oldId) => {
+    if (oldId) saveScrollPosition()
+    if (newId) restoreScrollPosition(newId)
+  }
+)
+
 watch(
   () => messages.value.length,
   () => scrollToBottom()