Split AdminDialogs.vue into per-dialog components

Break the monolithic admin dialog component into a thin dispatcher plus
one component per dialog type under admin/dialogs/, with a shared
AdminDialog frame (Modal wrapper, title, error and Cancel/Save actions).
No functional change. Also drop two unused input refs (nameInput,
displayNameInput).

frontend/src/admin/dialogs/RoleCreateDialog.vue

@@ -0,0 +1,23 @@
+<script setup>
+import AdminDialog from './AdminDialog.vue'
+
+defineProps({
+  dialog: { type: Object, required: true }
+})
+
+defineEmits(['submit', 'close'])
+</script>
+
+<template>
+  <AdminDialog
+    title="Create Role"
+    :busy="dialog.busy"
+    :error="dialog.error"
+    @submit="$emit('submit')"
+    @close="$emit('close')"
+  >
+    <label>Role Name
+      <input v-model="dialog.data.name" placeholder="Role name" required />
+    </label>
+  </AdminDialog>
+</template>