Adds settle receivable functionality

Implements a "Settle Receivable" feature for super users to record manual payments from users who owe money.

Introduces a dialog for inputting payment details (amount, method, description, reference), triggers an API call to record the transaction, and updates user balances and transaction history.

This is for non-lightning payments like cash, bank transfers, or checks.

static/js/index.js

@@ -82,6 +82,17 @@ window.app = Vue.createApp({
         discrepancies: null,
         checking: false,
         showDiscrepancies: false
+      },
+      settleReceivableDialog: {
+        show: false,
+        user_id: '',
+        username: '',
+        maxAmount: 0,
+        amount: 0,
+        payment_method: 'cash',
+        description: '',
+        reference: '',
+        loading: false
       }
     }
   },
@@ -852,6 +863,56 @@ window.app = Vue.createApp({
       this.receivableDialog.reference = ''
       this.receivableDialog.currency = null
     },
+    showSettleReceivableDialog(userBalance) {
+      // Only show for users who owe castle (negative balance)
+      if (userBalance.balance >= 0) return
+
+      this.settleReceivableDialog = {
+        show: true,
+        user_id: userBalance.user_id,
+        username: userBalance.username,
+        maxAmount: Math.abs(userBalance.balance), // Convert negative to positive
+        amount: Math.abs(userBalance.balance), // Default to full amount
+        payment_method: 'cash',
+        description: `Payment from ${userBalance.username}`,
+        reference: '',
+        loading: false
+      }
+    },
+    async submitSettleReceivable() {
+      this.settleReceivableDialog.loading = true
+      try {
+        const response = await LNbits.api.request(
+          'POST',
+          '/castle/api/v1/receivables/settle',
+          this.g.user.wallets[0].adminkey,
+          {
+            user_id: this.settleReceivableDialog.user_id,
+            amount: this.settleReceivableDialog.amount,
+            payment_method: this.settleReceivableDialog.payment_method,
+            description: this.settleReceivableDialog.description,
+            reference: this.settleReceivableDialog.reference || null
+          }
+        )
+
+        this.$q.notify({
+          type: 'positive',
+          message: response.data.message,
+          timeout: 3000
+        })
+
+        this.settleReceivableDialog.show = false
+
+        // Reload balances
+        await this.loadBalance()
+        await this.loadTransactions()
+        await this.loadAllUserBalances()
+      } catch (error) {
+        LNbits.utils.notifyApiError(error)
+      } finally {
+        this.settleReceivableDialog.loading = false
+      }
+    },
     formatSats(amount) {
       return new Intl.NumberFormat().format(amount)
     },