fix(settings): pin the settings and admin sidebars on scroll

The desktop section nav in both shells used a single sticky wrapper at
top-20. Because the nav can run taller than the scroll viewport and the
top offset assumed a topbar that sits outside the scroll region, the
panel drifted with the content and re-pinned abruptly on scroll-up.

Cap the wrapper height to the viewport and let it scroll independently
(top-6, max-h-[calc(100dvh-5.5rem)], overflow-y-auto, no-scrollbar) so
the nav stays pinned. Covers every /settings/* and /admin/* page through
the shared shells.

src/components/admin/admin-shell.tsx

@@ -316,7 +316,7 @@ export function AdminShell({ active, children }: AdminShellProps) {
           aria-label={t("admin.shell.sectionsNav")}
           className="hidden md:col-start-1 md:row-start-2 md:block"
         >
-          <div className="sticky top-20">
+          <div className="no-scrollbar sticky top-6 max-h-[calc(100dvh-5.5rem)] overflow-y-auto">
             <ul className="space-y-1">
               <li>
                 <Link

src/components/settings/settings-shell.tsx

@@ -502,7 +502,7 @@ export function SettingsShell({
           aria-label={t("settings.shell.sectionsNav")}
           className="hidden md:col-start-1 md:row-start-2 md:block"
         >
-          <div className="sticky top-20">
+          <div className="no-scrollbar sticky top-6 max-h-[calc(100dvh-5.5rem)] overflow-y-auto">
             <ul className="space-y-1">
               {visibleSections.map((section) => {
                 const isActive = section.slug === activeSlug;