feat(10-10): wire OsDragOverlay into App.vue; expose handleOsDrop on FileManagerView

- App.vue: import + mount <OsDragOverlay @files-dropped="onOsFilesDropped" />
  after <ToastContainer /> (overlay z-[9998] stays below toast z-[9999])
- App.vue: add onOsFilesDropped handler -> routeViewRef.value?.handleOsDrop?.(files)
- FileManagerView: extend defineExpose to include handleOsDrop delegating to
  existing onFilesSelected({ files, autoClassify: true }) — no duplicate upload logic
- Full test suite: 198 passed, 0 failures

frontend/src/App.vue

@@ -7,6 +7,7 @@
     </main>
   </div>
   <ToastContainer />
+  <OsDragOverlay @files-dropped="onOsFilesDropped" />
 </template>
 
 <script setup>
@@ -15,12 +16,17 @@ import { useRoute } from 'vue-router'
 import AppSidebar from './components/layout/AppSidebar.vue'
 import AuthLayout from './layouts/AuthLayout.vue'
 import ToastContainer from './components/ui/ToastContainer.vue'
+import OsDragOverlay from './components/layout/OsDragOverlay.vue'
 import { useTopicsStore } from './stores/topics.js'
 
 const route = useRoute()
 const topicsStore = useTopicsStore()
 const routeViewRef = ref(null)
 
+function onOsFilesDropped(files) {
+  routeViewRef.value?.handleOsDrop?.(files)
+}
+
 function onKeydown(e) {
   const tag = document.activeElement?.tagName
   if (['INPUT', 'TEXTAREA', 'SELECT'].includes(tag) || document.activeElement?.isContentEditable) return

frontend/src/views/FileManagerView.vue

@@ -186,5 +186,6 @@ defineExpose({
   triggerUpload: () => browserRef.value?.triggerUpload?.(),
   startNewFolder: () => browserRef.value?.startNewFolder?.(),
   clearSearch: () => browserRef.value?.clearSearch?.(),
+  handleOsDrop: (files) => onFilesSelected({ files, autoClassify: true }),
 })
 </script>