refactor(ui): full-width header with framed chat shell

Header now spans the full window width above the [sidebar | chat | right-sidebar] row instead of nesting inside the central column. The chat area becomes a self-contained framed window with its own border and rounded corners on all four sides, and sidebars sit flush against the header sharing its bg-sidebar so the seam is invisible.

Removed the duplicated shell controls the old layout needed to fake header-height inside sidebars: portal host on RightSidebar, paddingTop reservation, top drag overlay, duplicated layout-left / chat-new buttons in SidebarHeader, the showDesktopSidebarChrome block in SessionSidebar, and the conditional traffic-lights inset on the desktop header. Mac WCO inset now lives only on the header.

Moved the new-session action into the SessionSwitcher dropdown as its first item, removed the standalone chat-new button from the header, relocated scheduled-tasks into the left action group of the sidebar header, and bumped ContextPanel tab strip to h-10 to balance the more prominent header.

packages/ui/src/components/layout/ContextPanel.tsx

@@ -1828,7 +1828,7 @@ export const ContextPanel: React.FC = () => {
   const isFileTabActive = activeTab?.mode === 'file';
 
   const header = (
-    <header className="flex h-8 items-stretch border-b border-transparent">
+    <header className="flex h-10 items-stretch border-b border-transparent">
       <SortableTabsStrip
         items={tabItems}
         activeId={activeTab?.id ?? null}