Enables balance payments via invoice

Adds functionality for users to pay their Castle balance by generating and paying a Lightning invoice.
This includes:
- Adding API endpoints for invoice generation and payment recording.
- Updating the frontend to allow users to initiate the invoice payment process.
- Passing the wallet's `inkey` to the frontend for payment status checks.

static/js/index.js

@@ -34,6 +34,7 @@ window.app = Vue.createApp({
         amount: null,
         paymentRequest: null,
         paymentHash: null,
+        checkWalletKey: null,
         loading: false
       },
       settingsDialog: {
@@ -326,6 +327,7 @@ window.app = Vue.createApp({
         // Show the payment request in the dialog
         this.payDialog.paymentRequest = response.data.payment_request
         this.payDialog.paymentHash = response.data.payment_hash
+        this.payDialog.checkWalletKey = response.data.check_wallet_key
 
         this.$q.notify({
           type: 'positive',
@@ -334,21 +336,21 @@ window.app = Vue.createApp({
         })
 
         // Poll for payment completion
-        this.pollForPayment(response.data.payment_hash)
+        this.pollForPayment(response.data.payment_hash, response.data.check_wallet_key)
       } catch (error) {
         LNbits.utils.notifyApiError(error)
       } finally {
         this.payDialog.loading = false
       }
     },
-    async pollForPayment(paymentHash) {
+    async pollForPayment(paymentHash, checkWalletKey) {
       // Poll every 2 seconds for payment status
       const checkPayment = async () => {
         try {
           const response = await LNbits.api.request(
             'GET',
             `/api/v1/payments/${paymentHash}`,
-            this.g.user.wallets[0].inkey
+            checkWalletKey
           )
 
           if (response.data && response.data.paid) {
@@ -415,6 +417,7 @@ window.app = Vue.createApp({
       this.payDialog.amount = Math.abs(this.balance.balance)
       this.payDialog.paymentRequest = null
       this.payDialog.paymentHash = null
+      this.payDialog.checkWalletKey = null
       this.payDialog.show = true
     },
     async showReceivableDialog() {