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/asset/AssetReferences.vue

@@ -43,8 +43,8 @@
               </div>
               <Button
                 variant="ghost"
-                size="sm"
-                class="h-8 w-8 p-0 flex-shrink-0"
+                size="icon-sm"
+                class="flex-shrink-0"
                 @click="downloadFile(reference)"
               >
                 <Download class="h-4 w-4" />