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/task/columns.ts

@@ -143,8 +143,7 @@ export const createColumns = (callbacks?: ColumnCallbacks): ColumnDef<Task>[] =>
                     Button,
                     {
                       variant: 'outline',
-                      size: 'sm',
-                      class: 'h-8 w-8 p-0',
+                      size: 'icon-sm',
                     },
                     () => h(ChevronDown, { class: 'h-4 w-4' })
                   ),
@@ -334,8 +333,7 @@ export const createColumns = (callbacks?: ColumnCallbacks): ColumnDef<Task>[] =>
                     Button,
                     {
                       variant: 'ghost',
-                      size: 'sm',
-                      class: 'h-8 w-8 p-0',
+                      size: 'icon-sm',
                       onMouseDown: (e: Event) => {
                         e.stopPropagation()
                       },