fix(desktop): show every pinned session, however many there are

Pinned was capped at half the viewport by its own nested scroller, so past
roughly a dozen pins the rest were reachable only by scrolling inside a
scroller — a pin you have to go hunting for isn't doing its job.

Drop the cap and let the section grow into the sidebar's existing scroll,
and stop virtualizing Pinned: virtualization needs a bounded viewport to
measure against, which is exactly what's being removed. No count badge, no
"show more" — pin as many as you want and they all render.

Also back-fill pins on the API-server list route, which was the one list
path still windowing purely on recency.

apps/desktop/src/app/chat/sidebar/index.tsx

@@ -1288,7 +1288,7 @@ export function ChatSidebar({
             {!trimmedQuery && (
               <SidebarSessionsSection
                 activeSessionId={activeSidebarSessionId}
-                contentClassName={cn('flex max-h-[50vh] flex-col gap-px rounded-lg pb-2 pt-1', GROUP_BODY)}
+                contentClassName="flex flex-col gap-px rounded-lg pb-2 pt-1"
                 dndSensors={dndSensors}
                 emptyState={<SidebarPinnedEmptyState />}
                 label={s.pinned}

apps/desktop/src/app/chat/sidebar/sessions-section.tsx

@@ -313,7 +313,11 @@ export function SidebarSessionsSection({
   // wasn't looking at — the drag that landed a row in the wrong slot.
   const sortableRowIds = useMemo(() => reorderableRowIds(flatRows), [flatRows])
 
+  // Pinned never virtualizes. Virtualization needs a bounded viewport to
+  // measure against, and Pinned deliberately has none — however many chats you
+  // pin, all of them render and the sidebar's own scroll carries the length.
   const flatVirtualized =
+    !pinned &&
     !showEmptyState &&
     !groups?.length &&
     !projectOverview?.length &&

gateway/platforms/api_server.py

@@ -3279,13 +3279,19 @@ class APIServerAdapter(BasePlatformAdapter):
             offset=offset,
             include_children=include_children,
             order_by_last_active=True,
+            # A pin means "always reachable", so a pinned conversation that has
+            # aged past the recency window is back-filled rather than dropped.
+            include_pinned=True,
         )
+        # Back-filled pins arrive PAST the limit, so counting them would report
+        # another page that doesn't exist. Only the recency window decides.
+        windowed = sum(1 for s in sessions if not s.get("pinned"))
         return web.json_response({
             "object": "list",
             "data": [self._session_response(s) for s in sessions],
             "limit": limit,
             "offset": offset,
-            "has_more": len(sessions) == limit,
+            "has_more": windowed >= limit,
         })
 
     async def _handle_create_session(self, request: "web.Request") -> "web.Response":