refactor(layout): remove the legacy mobile layout from MainLayout and Header

Phone viewports run the separate MobileApp shell (and a viewport crossing
now reloads into it), so the desktop layout's mobile branch was
unreachable: the drawer machinery, the full-screen secondaryView surface
switch (including the terminal/diagram desktop carve-out nothing could
trigger), the mobile header with its tab bar, the Cmd+number tab
shortcuts, the mobile quota panel, and the surface guard that reset
non-chat tabs. DrawerContext had no consumers left and is deleted.
Header drops from 2630 to 1853 lines; the desktop render is unchanged.

packages/ui/src/contexts/DrawerContext.tsx

@@ -1,29 +0,0 @@
-import React from 'react';
-import type { MotionValue } from 'motion/react';
-
-export interface DrawerContextValue {
-    leftDrawerOpen: boolean;
-    rightDrawerOpen: boolean;
-    toggleLeftDrawer: () => void;
-    toggleRightDrawer: () => void;
-    // Motion values for real-time drawer dragging
-    leftDrawerX: MotionValue<number>;
-    rightDrawerX: MotionValue<number>;
-    leftDrawerWidth: React.MutableRefObject<number>;
-    rightDrawerWidth: React.MutableRefObject<number>;
-    setMobileLeftDrawerOpen: (open: boolean) => void;
-    setRightSidebarOpen: (open: boolean) => void;
-}
-
-const DrawerContext = React.createContext<DrawerContextValue | null>(null);
-
-export const DrawerProvider: React.FC<{
-    children: React.ReactNode;
-    value: DrawerContextValue;
-}> = ({ children, value }) => {
-    return (
-        <DrawerContext.Provider value={value}>
-            {children}
-        </DrawerContext.Provider>
-    );
-};