feat(web): agent auto-scroll (follow mode + jump-to-latest button) and bottom padding on terminal and agent

Agent (AcpPanel):
- Follow state: an isAtBottom ref + state. A scroll listener on
  the ScrollArea viewport keeps them in sync. The auto-scroll
  effect only fires scrollIntoView when the ref is true, so the
  user can scroll up to read history without the panel snapping
  back to the bottom.
- Jump-to-latest button: when !isAtBottom, an absolute-positioned
  '↓ Jump to latest' pill appears at the bottom of the ScrollArea;
  clicking it scrolls back to the bottom and re-enables follow.
- Bottom padding: messages container is now 'p-3 pb-6' and the
  trailing sentinel is 'h-8 shrink-0', so the last bubble has
  breathing room above the composer and isn't visually clipped.

Terminal (TerminalPanel):
- xterm container is now 'px-2 pt-2 pb-8' so the bottom row of
  the terminal isn't covered by the hint/status bar above the
  composer (or, in the standalone TerminalPanel, by the tab bar).

Both panels: extra bottom padding so the last line of content is
never hidden by an adjacent control.

web/src/components/acp/AcpPanel.tsx

@@ -164,23 +164,50 @@ export function AcpPanel({ workspaceId }: AcpPanelProps) {
     };
   }, []);
 
+  // Track "follow" state. If the user is near the bottom, new content
+  // pulls them along. If they scroll up, we stop following so they can
+  // read history; a back-to-bottom button appears. The ref lets the
+  // auto-scroll effect read the freshest value without depending on
+  // the state (which would cause a re-fire loop when scrollIntoView
+  // itself emits a scroll event).
+  const isAtBottomRef = useRef(true);
+  const [isAtBottom, setIsAtBottom] = useState(true);
+
   useEffect(() => {
     const root = scrollRootRef.current;
-    const sentinel = sentinelRef.current;
-    if (!root || !sentinel) return;
-
+    if (!root) return;
     const viewport = root.querySelector(
       "[data-radix-scroll-area-viewport]",
     ) as HTMLDivElement | null;
     if (!viewport) return;
+    const handler = () => {
+      const distance =
+        viewport.scrollHeight - viewport.scrollTop - viewport.clientHeight;
+      const atBottom = distance <= 80;
+      isAtBottomRef.current = atBottom;
+      setIsAtBottom(atBottom);
+    };
+    viewport.addEventListener("scroll", handler);
+    handler(); // sync initial state
+    return () => viewport.removeEventListener("scroll", handler);
+  }, []);
 
-    const distanceFromBottom =
-      viewport.scrollHeight - viewport.scrollTop - viewport.clientHeight;
-    if (distanceFromBottom <= 80) {
-      sentinel.scrollIntoView({ behavior: "smooth", block: "end" });
-    }
+  useEffect(() => {
+    const root = scrollRootRef.current;
+    const sentinel = sentinelRef.current;
+    if (!root || !sentinel) return;
+    if (!isAtBottomRef.current) return; // user scrolled up; let them read
+
+    sentinel.scrollIntoView({ behavior: "smooth", block: "end" });
   }, [coalesced, inflight?.text, localUserMessage]);
 
+  const scrollToBottom = useCallback(() => {
+    sentinelRef.current?.scrollIntoView({
+      behavior: "smooth",
+      block: "end",
+    });
+  }, []);
+
   const handleSend = () => {
     const content = input.trim();
     if (!content || !workspaceId || inflight) return;
@@ -255,8 +282,8 @@ export function AcpPanel({ workspaceId }: AcpPanelProps) {
         <StatusPill status={acpStatus} />
       </header>
 
-      <ScrollArea ref={scrollRootRef} className="flex-1">
-        <div className="flex flex-col gap-3 p-3">
+      <ScrollArea ref={scrollRootRef} className="relative flex-1">
+        <div className="flex flex-col gap-3 p-3 pb-6">
           {messages.length === 0 &&
             !localUserMessage &&
             !inflight &&
@@ -285,8 +312,20 @@ export function AcpPanel({ workspaceId }: AcpPanelProps) {
               {err.message}
             </div>
           ))}
-          <div ref={sentinelRef} />
+          {/* Sentinel: when auto-scrolled, the scroll lands here. h-8 gives
+              the last message a little breathing room above the composer. */}
+          <div ref={sentinelRef} className="h-8 shrink-0" />
         </div>
+        {!isAtBottom && (
+          <button
+            type="button"
+            onClick={scrollToBottom}
+            aria-label="Scroll to bottom"
+            className="absolute bottom-3 left-1/2 z-10 -translate-x-1/2 rounded-full border border-border bg-background/90 px-3 py-1 text-xs text-muted-foreground shadow-md backdrop-blur hover:bg-accent hover:text-foreground"
+          >
+            ↓ Jump to latest
+          </button>
+        )}
       </ScrollArea>
 
       <div className="flex h-12 shrink-0 items-center gap-2 border-t border-border bg-background px-3">

web/src/components/terminal/TerminalPanel.tsx

@@ -257,7 +257,7 @@ export function TerminalPanel({ workspaceId, shellId }: TerminalPanelProps) {
           )}
         </div>
       )}
-      <div ref={containerRef} className="min-h-0 flex-1 p-2" />
+      <div ref={containerRef} className="min-h-0 flex-1 px-2 pt-2 pb-8" />
     </section>
   );
 }