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/ui/command/index.ts

@@ -2,6 +2,7 @@ import type { Ref } from "vue"
 import { createContext } from "reka-ui"
 
 export { default as Command } from "./Command.vue"
+export { default as CheckableCommandItem } from "./CheckableCommandItem.vue"
 export { default as CommandDialog } from "./CommandDialog.vue"
 export { default as CommandEmpty } from "./CommandEmpty.vue"
 export { default as CommandGroup } from "./CommandGroup.vue"