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/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>