Adds receivable entry functionality

Implements the ability to record receivables (user owes the castle).

Adds API endpoint for creating receivable entries, which includes currency conversion to satoshis if fiat currency is provided.

Integrates a UI component (receivable dialog) for superusers to record debts owed by users, enhancing financial tracking capabilities.

templates/castle/index.html

@@ -153,6 +153,20 @@
               You must configure your wallet first
             </q-tooltip>
           </q-btn>
+          <q-btn
+            v-if="isSuperUser"
+            color="orange"
+            @click="showReceivableDialog"
+            :disable="!castleWalletConfigured"
+          >
+            Add Receivable
+            <q-tooltip v-if="!castleWalletConfigured">
+              Castle wallet must be configured first
+            </q-tooltip>
+            <q-tooltip v-else>
+              Record when a user owes the Castle
+            </q-tooltip>
+          </q-btn>
           <q-btn color="secondary" @click="loadTransactions">
             View Transactions
           </q-btn>
@@ -410,4 +424,82 @@
   </q-card>
 </q-dialog>
 
+<!-- Receivable Dialog -->
+<q-dialog v-model="receivableDialog.show" position="top">
+  <q-card v-if="receivableDialog.show" class="q-pa-lg q-pt-xl lnbits__dialog-card">
+    <q-form @submit="submitReceivable" class="q-gutter-md">
+      <div class="text-h6 q-mb-md">Add Receivable</div>
+
+      <q-select
+        filled
+        dense
+        v-model="receivableDialog.selectedUser"
+        :options="userOptions"
+        option-label="label"
+        option-value="value"
+        emit-value
+        map-options
+        label="User *"
+      ></q-select>
+
+      <q-input
+        filled
+        dense
+        v-model.trim="receivableDialog.description"
+        label="Description *"
+        placeholder="e.g., Room rental for 5 days"
+      ></q-input>
+
+      <q-select
+        filled
+        dense
+        v-model="receivableDialog.currency"
+        :options="currencyOptions"
+        option-label="label"
+        option-value="value"
+        emit-value
+        map-options
+        label="Currency"
+      ></q-select>
+
+      <q-input
+        filled
+        dense
+        v-model.number="receivableDialog.amount"
+        type="number"
+        :label="receivableAmountLabel"
+        min="0.01"
+        step="0.01"
+      ></q-input>
+
+      <q-select
+        filled
+        dense
+        v-model="receivableDialog.revenueAccount"
+        :options="revenueAccounts"
+        option-label="name"
+        option-value="id"
+        emit-value
+        map-options
+        label="Revenue Category *"
+      ></q-select>
+
+      <q-input
+        filled
+        dense
+        v-model.trim="receivableDialog.reference"
+        label="Reference (optional)"
+        placeholder="e.g., Invoice #456"
+      ></q-input>
+
+      <div class="row q-mt-lg">
+        <q-btn unelevated color="primary" type="submit" :loading="receivableDialog.loading">
+          Submit Receivable
+        </q-btn>
+        <q-btn v-close-popup flat color="grey" class="q-ml-auto">Cancel</q-btn>
+      </div>
+    </q-form>
+  </q-card>
+</q-dialog>
+
 {% endblock %}