feat: add marquee selection box overlay during drag-to-select

Show a semi-transparent blue rectangle overlay while dragging to
select rows, matching the project's primary color theme with dark
mode support. The box spans the full table width from drag start
to current mouse position.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/admin/AccountsView.vue

@@ -132,6 +132,7 @@
       </template>
       <template #table>
         <AccountBulkActionsBar :selected-ids="selIds" @delete="handleBulkDelete" @edit="showBulkEdit = true" @clear="selIds = []" @select-page="selectPage" @toggle-schedulable="handleBulkToggleSchedulable" />
+        <div ref="accountTableRef">
         <DataTable
           :columns="cols"
           :data="accounts"
@@ -252,6 +253,7 @@
             </div>
           </template>
         </DataTable>
+        </div>
       </template>
       <template #pagination><Pagination v-if="pagination.total > 0" :page="pagination.page" :total="pagination.total" :page-size="pagination.page_size" @update:page="handlePageChange" @update:pageSize="handlePageSizeChange" /></template>
     </TablePageLayout>
@@ -285,6 +287,7 @@ import { useAppStore } from '@/stores/app'
 import { useAuthStore } from '@/stores/auth'
 import { adminAPI } from '@/api/admin'
 import { useTableLoader } from '@/composables/useTableLoader'
+import { useSwipeSelect } from '@/composables/useSwipeSelect'
 import AppLayout from '@/components/layout/AppLayout.vue'
 import TablePageLayout from '@/components/layout/TablePageLayout.vue'
 import DataTable from '@/components/common/DataTable.vue'
@@ -319,6 +322,12 @@ const authStore = useAuthStore()
 const proxies = ref<Proxy[]>([])
 const groups = ref<AdminGroup[]>([])
 const selIds = ref<number[]>([])
+const accountTableRef = ref<HTMLElement | null>(null)
+useSwipeSelect(accountTableRef, {
+  isSelected: (id) => selIds.value.includes(id),
+  select: (id) => { if (!selIds.value.includes(id)) selIds.value.push(id) },
+  deselect: (id) => { selIds.value = selIds.value.filter(x => x !== id) }
+})
 const selPlatforms = computed<AccountPlatform[]>(() => {
   const platforms = new Set(
     accounts.value