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.

templates/castle/index.html

@@ -162,7 +162,8 @@
           :rows="allUserBalances"
           :columns="[
             {name: 'user', label: 'User', field: 'username', align: 'left'},
-            {name: 'balance', label: 'Amount Owed', field: 'balance', align: 'right'}
+            {name: 'balance', label: 'Amount Owed', field: 'balance', align: 'right'},
+            {name: 'actions', label: 'Actions', align: 'center'}
           ]"
           row-key="user_id"
           hide-pagination
@@ -189,6 +190,21 @@
               </div>
             </q-td>
           </template>
+          <template v-slot:body-cell-actions="props">
+            <q-td :props="props">
+              <q-btn
+                v-if="props.row.balance < 0"
+                flat
+                dense
+                size="sm"
+                color="primary"
+                icon="payments"
+                @click="showSettleReceivableDialog(props.row)"
+              >
+                <q-tooltip>Settle receivable (user pays)</q-tooltip>
+              </q-btn>
+            </q-td>
+          </template>
         </q-table>
       </q-card-section>
     </q-card>
@@ -1108,4 +1124,83 @@
   </q-card>
 </q-dialog>
 
+<!-- Settle Receivable Dialog (Super User Only) -->
+<q-dialog v-model="settleReceivableDialog.show" position="top">
+  <q-card class="q-pa-md" style="min-width: 400px">
+    <q-form @submit="submitSettleReceivable">
+      <h6 class="q-my-none q-mb-md">Settle Receivable</h6>
+
+      <div class="q-mb-md">
+        <div class="text-subtitle2">User</div>
+        <div>{% raw %}{{ settleReceivableDialog.username }}{% endraw %}</div>
+        <div class="text-caption text-grey">{% raw %}{{ settleReceivableDialog.user_id }}{% endraw %}</div>
+      </div>
+
+      <div class="q-mb-md">
+        <div class="text-subtitle2">Amount Owed</div>
+        <div class="text-negative text-h6">{% raw %}{{ formatSats(settleReceivableDialog.maxAmount) }}{% endraw %} sats</div>
+      </div>
+
+      <q-input
+        filled
+        dense
+        v-model.number="settleReceivableDialog.amount"
+        type="number"
+        label="Settlement Amount (sats) *"
+        hint="Amount user is paying (max: owed amount)"
+        :max="settleReceivableDialog.maxAmount"
+        :rules="[
+          val => val !== null && val !== undefined && val !== '' || 'Amount is required',
+          val => val > 0 || 'Amount must be positive',
+          val => val <= settleReceivableDialog.maxAmount || 'Cannot exceed owed amount'
+        ]"
+      ></q-input>
+
+      <q-select
+        filled
+        dense
+        v-model="settleReceivableDialog.payment_method"
+        :options="[
+          {label: 'Cash', value: 'cash'},
+          {label: 'Bank Transfer', value: 'bank_transfer'},
+          {label: 'Check', value: 'check'},
+          {label: 'Other', value: 'other'}
+        ]"
+        option-label="label"
+        option-value="value"
+        emit-value
+        map-options
+        label="Payment Method *"
+        :rules="[val => !!val || 'Payment method is required']"
+      ></q-select>
+
+      <q-input
+        filled
+        dense
+        v-model="settleReceivableDialog.description"
+        type="text"
+        label="Description *"
+        hint="Description of the payment"
+        :rules="[val => !!val || 'Description is required']"
+      ></q-input>
+
+      <q-input
+        filled
+        dense
+        v-model="settleReceivableDialog.reference"
+        type="text"
+        label="Reference (optional)"
+        hint="Receipt number, transaction ID, etc."
+      ></q-input>
+
+      <div class="row q-mt-md q-gutter-sm">
+        <q-btn unelevated color="primary" type="submit" :loading="settleReceivableDialog.loading">
+          Settle Receivable
+        </q-btn>
+        <q-btn v-close-popup flat color="grey">Cancel</q-btn>
+      </div>
+    </q-form>
+  </q-card>
+</q-dialog>
+
 {% endblock %}