Phase 3: Unify controls across shot/asset/task toolbars and detail panels

Extracts duplication that built up as shot/asset/task features reached
parity: CheckableCommandItem/ColumnToggleList replace 14+ hand-rolled
checkbox-list blocks, shared toolbar pieces (debounced search, detail-panel
toggle, clear-filters, segmented view/context toggle) replace copy-pasted
markup in the three table toolbars, icon-only buttons standardize on the
icon-sm size, TaskBulkActionsMenu's Assign To submenu matches Set Status,
and a shared DetailPanelOverlay/Header/Loading/Error shell backs all three
detail panels (adding a previously-missing error state to the task panel).

frontend/src/components/shared/DetailPanelHeader.vue

@@ -0,0 +1,27 @@
+<script setup lang="ts">
+import { X } from 'lucide-vue-next'
+import { Button } from '@/components/ui/button'
+
+interface Props {
+  title: string
+  deletedAt?: string | null
+}
+
+defineProps<Props>()
+defineEmits<{ close: [] }>()
+</script>
+
+<template>
+  <div class="p-6 border-b">
+    <div class="flex items-center justify-between gap-3">
+      <div class="flex items-center gap-3 min-w-0 flex-1">
+        <h2 class="text-xl font-bold truncate" :class="{ 'line-through text-muted-foreground': deletedAt }">{{ title }}</h2>
+        <slot name="badges" />
+      </div>
+
+      <Button variant="ghost" size="icon-sm" class="flex-shrink-0" @click="$emit('close')">
+        <X class="h-4 w-4" />
+      </Button>
+    </div>
+  </div>
+</template>