fix(sidebar): keep the Watching list inside its own scroll slot

The 'Watching' block is the only sidebar flex child that sets `min-h-0`,
so it is the only one the column is allowed to squeeze below its content
height. With a longer watchlist on a short viewport it collapsed while the
list kept painting outside the box, covering the Help / API docs / About /
Terms links and the sponsor card.

Give the list its own scroll container and keep the section label pinned.

components/shell/Sidebar.tsx

@@ -54,12 +54,12 @@ const Sidebar = ({ user, watchlist }: SidebarProps) => {
                 </Link>
             </nav>
 
-            <div className="flex flex-col gap-1 min-h-0">
-                <p className="side-label">Watching</p>
+            <div className="flex min-h-0 flex-col gap-1">
+                <p className="side-label shrink-0">Watching</p>
                 {watchlist.length === 0 ? (
                     <p className="px-2.5 text-[12.5px] text-faint">Star a stock to pin it here.</p>
                 ) : (
-                    <ul className="flex flex-col gap-0.5">
+                    <ul className="scrollbar-hide-default flex min-h-0 flex-col gap-0.5 overflow-y-auto">
                         {watchlist.map(({ symbol, company }) => (
                             <li key={symbol}>
                                 <Link