fix: improve "Load older" button styling and alignment

- Centered button using mx-auto w-fit wrapper (matches SystemMessage pattern)
- Changed variant from secondary to outline for subtler appearance
- Reduced visual weight with text-xs opacity-80 hover:opacity-100
- Added ↑ arrow icon before the text
- Adjusted spacing to py-1 mb-2 and gap-1.5

Fixes alignment issues where button wasn't centered with content below and improves visual consistency with page design.

web/src/components/AssistantChat/HappyThread.tsx

@@ -267,14 +267,15 @@ export function HappyThread(props: {
                                     ) : null}
 
                                     {props.hasMoreMessages && !props.isLoadingMessages ? (
-                                        <div className="mb-3">
+                                        <div className="py-1 mb-2">
+                                            <div className="mx-auto w-fit">
                                                 <Button
-                                                variant="secondary"
+                                                    variant="outline"
                                                     size="sm"
                                                     onClick={handleLoadMore}
                                                     disabled={props.isLoadingMoreMessages || props.isLoadingMessages}
                                                     aria-busy={props.isLoadingMoreMessages}
-                                                className="gap-2"
+                                                    className="gap-1.5 text-xs opacity-80 hover:opacity-100"
                                                 >
                                                     {props.isLoadingMoreMessages ? (
                                                         <>
@@ -282,10 +283,14 @@ export function HappyThread(props: {
                                                             Loading…
                                                         </>
                                                     ) : (
-                                                    'Load older'
+                                                        <>
+                                                            <span aria-hidden="true">↑</span>
+                                                            Load older
+                                                        </>
                                                     )}
                                                 </Button>
                                             </div>
+                                        </div>
                                     ) : null}
 
                                     {import.meta.env.DEV && props.normalizedMessagesCount === 0 && props.rawMessagesCount > 0 ? (