fix(app): store NWC wallet secret in encrypted vault instead of plaintext localStorage

Replace localStorage.getItem/setItem with storeApiKey/getApiKey/deleteApiKey
from key-vault. Make loadConnection(), connect(), and disconnect() async.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/composables/useNWC.ts

@@ -1,6 +1,7 @@
 import { ref } from 'vue'
+import { storeApiKey, getApiKey, deleteApiKey } from '@/utils/key-vault'
 
-const STORAGE_KEY = 'aiui-nwc-connection'
+const VAULT_PROVIDER = 'nwc'
 
 export interface NWCConnection {
   relayUrl: string
@@ -12,9 +13,9 @@ const isConnected = ref(false)
 const balance = ref<number | null>(null)
 const connectionString = ref<string | null>(null)
 
-function loadConnection(): NWCConnection | null {
+async function loadConnection(): Promise<NWCConnection | null> {
   try {
-    const stored = localStorage.getItem(STORAGE_KEY)
+    const stored = await getApiKey(VAULT_PROVIDER)
     if (stored) {
       connectionString.value = stored
       isConnected.value = true
@@ -39,25 +40,25 @@ function parseConnectionString(str: string): NWCConnection | null {
 }
 
 export function useNWC() {
-  function connect(nwcString: string): boolean {
+  async function connect(nwcString: string): Promise<boolean> {
     const parsed = parseConnectionString(nwcString)
     if (!parsed) return false
 
-    localStorage.setItem(STORAGE_KEY, nwcString)
+    await storeApiKey(VAULT_PROVIDER, nwcString)
     connectionString.value = nwcString
     isConnected.value = true
     return true
   }
 
-  function disconnect() {
-    localStorage.removeItem(STORAGE_KEY)
+  async function disconnect() {
+    await deleteApiKey(VAULT_PROVIDER)
     connectionString.value = null
     isConnected.value = false
     balance.value = null
   }
 
   async function getBalance(): Promise<number | null> {
-    const conn = loadConnection()
+    const conn = await loadConnection()
     if (!conn) return null
 
     // NIP-47: Send get_balance request via relay
@@ -66,7 +67,7 @@ export function useNWC() {
   }
 
   async function payInvoice(bolt11: string): Promise<{ success: boolean; preimage?: string; error?: string }> {
-    const conn = loadConnection()
+    const conn = await loadConnection()
     if (!conn) return { success: false, error: 'Not connected' }
 
     // NIP-47: Send pay_invoice request via relay