Mass-select grid and bulk delete for moderators

- Select mode in the filter row (upload.delete permission): tiles
  toggle selection in the capture phase, a floating action bar
  offers select-all and delete with confirmation popover
- New POST /api/uploads/bulk-delete endpoint (validated, deduped,
  max 200 ids) removing rows, files and attachment data
- upload-deleted broadcast on bulk and single delete; other clients
  refresh the grid and drop matching notifications
- Also: skeleton tile grid as loading state, dropzone logic moved
  to the layout-level GlobalDropZone, obsolete ytimg preconnect
  removed

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/composables/useNotifications.ts

@@ -49,10 +49,17 @@ export function useNotifications() {
     await $fetch(`/api/notifications/${id}/read`, { method: 'POST' })
   }
 
+  function removeByUploads(uploadIds: string[]) {
+    const removed = notifications.value.filter(n => uploadIds.includes(n.uploadId))
+    if (!removed.length) return
+    notifications.value = notifications.value.filter(n => !uploadIds.includes(n.uploadId))
+    unreadCount.value = Math.max(0, unreadCount.value - removed.filter(n => !n.isRead).length)
+  }
+
   function reset() {
     notifications.value = []
     unreadCount.value = 0
   }
 
-  return { notifications, unreadCount, unreadUploadIds, fetchAll, addLive, markAllRead, markRead, reset }
+  return { notifications, unreadCount, unreadUploadIds, fetchAll, addLive, markAllRead, markRead, removeByUploads, reset }
 }