fix(chat): rest the jump-to-latest button on the composer, not the padded scroller

The button was a sticky child of the scroll container, which reserves
composer-sized bottom padding. Sticky boxes are clamped by their
containing block's content box, so the 12px offset never applied and the
button parked 160px above the composer, on top of the transcript.

Anchor it to the composer wrapper's own top edge instead, so the offset
no longer depends on the --composer-reserve constant.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/page.tsx

@@ -3466,19 +3466,6 @@ export default function Home() {
               {activeError && <p className="error-message" role="alert">{activeError}</p>}
             </div>
             )}
-            {jumpToLatestVisible && (
-              <div className="pointer-events-none sticky bottom-3 z-10 flex h-0 items-end justify-center">
-                <button
-                  className="pointer-events-auto inline-flex min-h-11 min-w-11 items-center gap-1.5 rounded-full border border-border bg-canvas px-4 text-sm text-ink shadow-md transition-colors outline-none hover:bg-canvas-muted focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50"
-                  type="button"
-                  aria-label="跳到最新"
-                  onClick={conversationAnchor.anchorToLatest}
-                >
-                  <ArrowDown aria-hidden="true" className="size-4" />
-                  跳到最新
-                </button>
-              </div>
-            )}
           </div>
         )}
 
@@ -3504,6 +3491,19 @@ export default function Home() {
         )}
 
         {!rectificationSurfaceOpen && !onboardingFormActive && <div className={`composer-wrap ${starterHomeVisible ? "composer-wrap-starter" : ""}`}>
+          {jumpToLatestVisible && (
+            <div className="pointer-events-none absolute inset-x-0 bottom-full flex justify-center pb-3">
+              <button
+                className="pointer-events-auto inline-flex min-h-11 min-w-11 items-center gap-1.5 rounded-full border border-border bg-canvas px-4 text-sm text-ink shadow-md transition-colors outline-none hover:bg-canvas-muted focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50"
+                type="button"
+                aria-label="跳到最新"
+                onClick={conversationAnchor.anchorToLatest}
+              >
+                <ArrowDown aria-hidden="true" className="size-4" />
+                跳到最新
+              </button>
+            </div>
+          )}
           {activeSuggestions.length > 0 && (
             <div className="composer-suggestions" aria-label="推荐继续提问">
               {activeSuggestions.map((question) => (