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/composables/useDebouncedSearch.ts

@@ -0,0 +1,11 @@
+export function useDebouncedSearch(onSearch: (value: string) => void, delay = 300) {
+  let timeout: ReturnType<typeof setTimeout> | null = null
+
+  const debouncedSearch = (value: string | number) => {
+    const searchValue = typeof value === 'string' ? value : String(value)
+    if (timeout) clearTimeout(timeout)
+    timeout = setTimeout(() => onSearch(searchValue), delay)
+  }
+
+  return { debouncedSearch }
+}