feat(web): load older history by scrolling to top

Remove the redundant "Load older" button; the existing top sentinel
already auto-loads older pages when approaching the top. Loading state
moves to a floating pill overlay so prepends no longer shift layout.

web/src/components/AssistantChat/HappyThread.tsx

@@ -915,6 +915,15 @@ export function HappyThread(props: {
                         <span>{t('misc.loadingMessages')}</span>
                     </div>
                 ) : null}
+                {props.isLoadingMoreMessages && !props.isSyncingTail ? (
+                    <div
+                        role="status"
+                        className="pointer-events-none absolute left-1/2 top-3 z-20 flex -translate-x-1/2 items-center gap-1.5 rounded-full border border-[var(--app-border)] bg-[var(--app-bg)]/90 px-2.5 py-1 text-xs text-[var(--app-hint)] shadow-sm backdrop-blur"
+                    >
+                        <Spinner size="sm" label={null} className="text-current" />
+                        <span>{t('misc.loading')}</span>
+                    </div>
+                ) : null}
                 <ThreadPrimitive.Viewport
                     asChild
                     autoScroll={false}
@@ -935,35 +944,6 @@ export function HappyThread(props: {
                                         </div>
                                     ) : null}
 
-                                    {props.hasMoreMessages && !props.isSyncingTail ? (
-                                        <div className="py-1 mb-2">
-                                            <div className="mx-auto w-fit">
-                                                <Button
-                                                    variant="outline"
-                                                    size="sm"
-                                                    onClick={() => {
-                                                        void loadOlderFromUserAction()
-                                                    }}
-                                                    disabled={props.isLoadingMoreMessages || props.isSyncingTail}
-                                                    aria-busy={props.isLoadingMoreMessages}
-                                                    className="gap-1.5 text-xs opacity-80 hover:opacity-100"
-                                                >
-                                                    {props.isLoadingMoreMessages ? (
-                                                        <>
-                                                            <Spinner size="sm" label={null} className="text-current" />
-                                                            {t('misc.loading')}
-                                                        </>
-                                                    ) : (
-                                                        <>
-                                                            <span aria-hidden="true">↑</span>
-                                                            {t('misc.loadOlder')}
-                                                        </>
-                                                    )}
-                                                </Button>
-                                            </div>
-                                        </div>
-                                    ) : null}
-
                                     {import.meta.env.DEV && props.normalizedMessagesCount === 0 && props.rawMessagesCount > 0 ? (
                                         <div className="mb-2 rounded-md bg-amber-500/10 p-2 text-xs">
                                             Message normalization returned 0 items for {props.rawMessagesCount} messages (see `web/src/chat/normalize.ts`).

web/src/lib/locales/en.ts

@@ -848,7 +848,6 @@ export default {
   'misc.fastModeFast': 'Fast',
   'misc.variant': 'Variant',
   'misc.loading': 'Loading…',
-  'misc.loadOlder': 'Load older',
   'misc.newMessage': '{n} new message{s}',
   'misc.loadingMessages': 'Loading messages…',
   'misc.typeMessage': "Type 'continue' to resume...",

web/src/lib/locales/zh-CN.ts

@@ -852,7 +852,6 @@ export default {
   'misc.fastModeFast': '快速',
   'misc.variant': '变体',
   'misc.loading': '加载中…',
-  'misc.loadOlder': '加载更早的',
   'misc.newMessage': '{n} 条新消息',
   'misc.loadingMessages': '加载消息中…',
   'misc.typeMessage': "输入 'continue' 继续...",