feat(nostr): Add Nostr relay connection and status management

- Integrate nostr-tools for Nostr relay connectivity
- Create NostrClient for managing relay connections
- Implement useNostr composable for reactive connection handling
- Add ConnectionStatus component to display relay connection state
- Configure environment variable for Nostr relay endpoints
- Update App.vue to manage Nostr connection lifecycle

src/composables/useNostr.ts

@@ -0,0 +1,32 @@
+import { ref } from 'vue'
+import { NostrClient, type NostrClientConfig } from '@/lib/nostr/client'
+
+export function useNostr(config: NostrClientConfig) {
+  const client = new NostrClient(config)
+  const isConnected = ref(false)
+  const error = ref<Error | null>(null)
+
+  async function connect() {
+    try {
+      error.value = null
+      await client.connect()
+      isConnected.value = client.isConnected
+    } catch (err) {
+      error.value = err instanceof Error ? err : new Error('Failed to connect')
+      isConnected.value = false
+    }
+  }
+
+  function disconnect() {
+    client.disconnect()
+    isConnected.value = false
+    error.value = null
+  }
+
+  return {
+    isConnected,
+    error,
+    connect,
+    disconnect
+  }
+}