feat(app): add empty states for sidebar, sessions, and open-project

Sidebar shows a card with "No projects yet" and an Add project ghost
button. Sessions screen shows "No sessions yet" with a ghost Back
button. Open-project screen uses MenuHeader borderless, shared Button,
and shows introductory text when no projects exist.

packages/app/src/components/left-sidebar.tsx

@@ -520,6 +520,7 @@ function MobileSidebar({
                 isRefreshing={isManualRefresh && isRevalidating}
                 onRefresh={handleRefresh}
                 onWorkspacePress={closeToAgent}
+                onAddProject={handleOpenProject}
                 parentGestureRef={closeGestureRef}
               />
             )}
@@ -701,6 +702,7 @@ function DesktopSidebar({
           projects={projects}
           isRefreshing={isManualRefresh && isRevalidating}
           onRefresh={handleRefresh}
+          onAddProject={handleOpenProject}
         />
       )}