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/components/UploadTile.vue

@@ -14,7 +14,11 @@ interface TileUpload {
   commentCount?: number
 }
 
-const props = defineProps<{ upload: TileUpload }>()
+const props = withDefaults(defineProps<{ upload: TileUpload, selectable?: boolean, selected?: boolean }>(), {
+  selectable: false,
+  selected: false,
+})
+const emit = defineEmits<{ select: [] }>()
 const { t } = useI18n()
 
 const { unreadUploadIds } = useNotifications()
@@ -47,7 +51,17 @@ const spanClass = computed(() => {
 // 1. Ladeindikator oben sofort starten
 // 2. Thread-Daten + Bild/Video vorladen (min. 600 ms, damit der Indikator sichtbar läuft)
 // 3. Wenn geladen: viewTransitionName setzen und via startViewTransition navigieren
+// Capture-Phase: läuft vor NuxtLinks eigenem Klick-Handler — preventDefault +
+// stopImmediatePropagation verhindert die Navigation sicher in allen Browsern
+function captureClick(e: MouseEvent) {
+  if (!props.selectable) return
+  e.preventDefault()
+  e.stopImmediatePropagation()
+  emit('select')
+}
+
 async function openUpload(e: MouseEvent) {
+  if (props.selectable) return
   if (e.metaKey || e.ctrlKey || e.shiftKey || e.altKey) return
   e.preventDefault()
   if (navigating.value) return
@@ -116,10 +130,11 @@ async function openUpload(e: MouseEvent) {
     :to="`/thread/${upload.id}`"
     class="upload-tile group block"
     :class="[spanClass, navigating ? 'opacity-80' : '']"
+    @click.capture="captureClick"
     @click="openUpload"
   >
     <div
-      :class="cn('upload-tile-media relative h-full w-full overflow-hidden rounded-md', hasNotification && 'ring-4 ring-primary')"
+      :class="cn('upload-tile-media relative h-full w-full overflow-hidden rounded-md', selected ? 'ring-4 ring-accent' : hasNotification && 'ring-4 ring-primary')"
       :style="lastViewedId === upload.id ? 'view-transition-name: upload-media' : undefined"
     >
       <NuxtImg
@@ -145,6 +160,12 @@ async function openUpload(e: MouseEvent) {
         <Icon name="ph:music-note-fill" v-if="upload.type === 'audio'" class="size-8 text-muted-foreground" />
         <Icon name="ph:link-fill" v-else class="size-8 text-muted-foreground" />
       </div>
+      <span
+        v-if="selectable"
+        :class="cn('absolute top-1.5 left-1.5 z-10 flex size-6 items-center justify-center rounded-full border shadow', selected ? 'border-accent bg-accent text-accent-foreground' : 'border-muted-foreground/40 bg-background/60')"
+      >
+        <Icon v-if="selected" name="ph:check-bold" class="size-3.5" />
+      </span>
       <span
         v-if="hasNotification"
         class="absolute top-1.5 right-1.5 z-10 flex size-6 items-center justify-center rounded-full bg-accent text-accent-foreground shadow"