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/components/common/DataTable.vue

@@ -152,6 +152,7 @@
           v-else
           v-for="(row, index) in sortedData"
           :key="resolveRowKey(row, index)"
+          :data-row-id="resolveRowKey(row, index)"
           class="hover:bg-gray-50 dark:hover:bg-dark-800"
         >
           <td