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/shot/ShotCard.vue

@@ -21,7 +21,7 @@
         <!-- Actions Menu -->
         <DropdownMenu>
           <DropdownMenuTrigger asChild @click.stop>
-            <Button variant="ghost" size="sm" class="h-8 w-8 p-0 opacity-0 group-hover:opacity-100 transition-opacity">
+            <Button variant="ghost" size="icon-sm" class="opacity-0 group-hover:opacity-100 transition-opacity">
               <MoreHorizontal class="h-4 w-4" />
               <span class="sr-only">Shot actions</span>
             </Button>