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/project/ProjectMemberManagement.vue

@@ -83,7 +83,7 @@
           <!-- Actions -->
           <DropdownMenu>
             <DropdownMenuTrigger as-child>
-              <Button variant="ghost" size="sm" class="h-8 w-8 p-0">
+              <Button variant="ghost" size="icon-sm">
                 <MoreHorizontal class="h-4 w-4" />
               </Button>
             </DropdownMenuTrigger>