fix: enable sidebar expand and project switching on mobile

- Sidebar now uses overlay pattern on mobile (fixed position when open)
- Added backdrop overlay that dismisses sidebar on tap
- Made collapse toggle button visible on all screen sizes
- Made project options menu visible on all screen sizes

Previously the sidebar was forced to collapsed width (w-16) on mobile
even when sidebarOpen was true, and the toggle/options buttons were
hidden with `hidden lg:flex`.

apps/ui/src/components/layout/sidebar/components/collapse-toggle-button.tsx

@@ -17,9 +17,7 @@ export function CollapseToggleButton({
     <button
       onClick={toggleSidebar}
       className={cn(
-        // Show on desktop always, show on mobile only when sidebar is open
-        sidebarOpen ? 'flex' : 'hidden lg:flex',
-        'absolute top-[68px] -right-3 z-9999',
+        'flex absolute top-[68px] -right-3 z-9999',
         'group/toggle items-center justify-center w-7 h-7 rounded-full',
         // Glass morphism button
         'bg-card/95 backdrop-blur-sm border border-border/80',