Enables user selection for permissions

Replaces the user ID input field with a user selection dropdown,
allowing administrators to search and select users for permission
management. This simplifies the process of assigning permissions
and improves user experience.

Fetches Castle users via a new API endpoint and filters them
based on search input. Only users with Castle accounts
(receivables, payables, equity, or permissions) are listed.

templates/castle/permissions.html

@@ -187,19 +187,33 @@
     </q-card-section>
 
     <q-card-section class="q-gutter-md">
-      <!-- User ID -->
-      <q-input
+      <!-- User -->
+      <q-select
         v-model="grantForm.user_id"
-        label="User ID *"
-        hint="Wallet ID of the user"
+        label="User *"
+        hint="Search and select a user"
+        :options="userOptions"
+        option-value="id"
+        option-label="label"
+        emit-value
+        map-options
+        use-input
+        @filter="filterUsers"
         outlined
         dense
-        :rules="[val => !!val || 'User ID is required']"
+        :rules="[val => !!val || 'User is required']"
       >
         <template v-slot:prepend>
           <q-icon name="person"></q-icon>
         </template>
-      </q-input>
+        <template v-slot:no-option>
+          <q-item>
+            <q-item-section class="text-grey">
+              No users found
+            </q-item-section>
+          </q-item>
+        </template>
+      </q-select>
 
       <!-- Account -->
       <q-select