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/screens/sessions-screen.tsx

@@ -1,10 +1,14 @@
 import { useMemo, useState, useCallback, useEffect } from "react";
-import { View } from "react-native";
+import { View, Text } from "react-native";
 import { useIsFocused } from "@react-navigation/native";
+import { router } from "expo-router";
 import { StyleSheet } from "react-native-unistyles";
+import { ChevronLeft } from "lucide-react-native";
 import { MenuHeader } from "@/components/headers/menu-header";
+import { Button } from "@/components/ui/button";
 import { AgentList } from "@/components/agent-list";
 import { useAllAgentsList } from "@/hooks/use-all-agents-list";
+import { buildHostOpenProjectRoute } from "@/utils/host-routes";
 
 export function SessionsScreen({ serverId }: { serverId: string }) {
   const isFocused = useIsFocused();
@@ -44,13 +48,26 @@ function SessionsScreenContent({ serverId }: { serverId: string }) {
   return (
     <View style={styles.container}>
       <MenuHeader title="Sessions" />
-      <AgentList
-        agents={sortedAgents}
-        showCheckoutInfo={false}
-        isRefreshing={isManualRefresh && isRevalidating}
-        onRefresh={handleRefresh}
-        showAttentionIndicator={false}
-      />
+      {sortedAgents.length === 0 ? (
+        <View style={styles.emptyContainer}>
+          <Text style={styles.emptyText}>No sessions yet</Text>
+          <Button
+            variant="ghost"
+            leftIcon={ChevronLeft}
+            onPress={() => router.navigate(buildHostOpenProjectRoute(serverId) as any)}
+          >
+            Back
+          </Button>
+        </View>
+      ) : (
+        <AgentList
+          agents={sortedAgents}
+          showCheckoutInfo={false}
+          isRefreshing={isManualRefresh && isRevalidating}
+          onRefresh={handleRefresh}
+          showAttentionIndicator={false}
+        />
+      )}
     </View>
   );
 }
@@ -60,4 +77,15 @@ const styles = StyleSheet.create((theme) => ({
     flex: 1,
     backgroundColor: theme.colors.surface0,
   },
+  emptyContainer: {
+    flex: 1,
+    justifyContent: "center",
+    alignItems: "center",
+    gap: theme.spacing[6],
+    padding: theme.spacing[6],
+  },
+  emptyText: {
+    color: theme.colors.foregroundMuted,
+    fontSize: theme.fontSize.lg,
+  },
 }));