Implements expense approval workflow

Adds an admin approval workflow for user-submitted expenses. This ensures that only valid expenses affect user balances.

The workflow includes pending expense states, admin approval/rejection actions, balance filtering, and UI updates.

static/js/index.js

@@ -497,6 +497,7 @@ window.app = Vue.createApp({
     async loadPendingExpenses() {
       try {
         if (!this.isSuperUser) return
+        if (!this.g.user.wallets[0]?.adminkey) return
 
         const response = await LNbits.api.request(
           'GET',