perf+ux: cut grid re-renders, coalesce discard, dedup bulk mutations

Frontend cleanup pass driven by the post-shadcn review.

Performance
- Memoize PhotoThumbnail and route cell click/double-click through
  stable handlers so heap-membership invalidation no longer re-renders
  every visible thumbnail.
- Cap usePhotosQuery's eager background page-walk at 20 pages with a
  50ms inter-page yield — was unbounded (up to 100k photos cold).
- Drop the per-thumbnail loading spinner in favour of the existing
  pulse skeleton; only retry state still surfaces a spinner.

UX
- Coalesce rapid X/U presses into a single undo entry + one toast
  (1.2s window) so accidental bursts are easy to back out.
- Optimistic rating/color updates with per-id snapshot rollback on
  error, matching the existing discard pattern.
- Section-aware empty timeline state with a Clear-all-filters CTA.
- Carry the search-match chip from the grid into the preview header.
- Add a basket-icon badge for active heap membership so the green
  tint isn't the only signal (colorblind-safe).
- Standardise error toasts via formatApiError(): FastAPI detail,
  validation arrays, axios message, with a 'Network Error' filter.

Architecture
- Extract useBulkPhotoMutations and stop duplicating
  bulkRating/bulkColor across RightSidebar and useKeyboardShortcuts.
- Split RightSidebar (714 -> 448 LOC) and PhotoInfoPanel (952 -> 716)
  into co-located sub-components: BulkTakenAtEditor, BulkTagsEditor,
  TagsEditor, TakenAtEditor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/heaps/HeapsPanel.tsx

@@ -21,6 +21,7 @@ import { heaps as heapsApi, downloads, type Heap } from '../../services/api'
 import { useFilterStore } from '../../store/filterStore'
 import { toast } from '../ToastContainer'
 import { PHOTO_DRAG_MIME } from '../timeline/PhotoThumbnail'
+import { formatApiError } from '../../lib/apiError'
 import { HeapConvertDialog } from './HeapConvertDialog'
 import { ShareDialog } from '../sharing/ShareDialog'
 import { Input } from '@/components/ui/input'
@@ -79,7 +80,7 @@ export function HeapsPanel() {
       setCreating(false)
     },
     onError: (e: any) =>
-      toast.error('Failed to create heap', e.message || 'Unknown error'),
+      toast.error('Failed to create heap', formatApiError(e)),
   })
 
   const setActiveMutation = useMutation({
@@ -90,7 +91,7 @@ export function HeapsPanel() {
       toast.success('Active heap', `Now adding to "${heap.name}" with T`)
     },
     onError: (e: any) =>
-      toast.error('Failed to set active', e.message || 'Unknown error'),
+      toast.error('Failed to set active', formatApiError(e)),
   })
 
   const deleteMutation = useMutation({
@@ -103,7 +104,7 @@ export function HeapsPanel() {
       }
     },
     onError: (e: any) =>
-      toast.error('Failed to delete heap', e.message || 'Unknown error'),
+      toast.error('Failed to delete heap', formatApiError(e)),
   })
 
   const renameMutation = useMutation({
@@ -111,7 +112,7 @@ export function HeapsPanel() {
       heapsApi.update(heapId, { name }),
     onSuccess: () => invalidate(),
     onError: (e: any) =>
-      toast.error('Failed to rename heap', e.message || 'Unknown error'),
+      toast.error('Failed to rename heap', formatApiError(e)),
   })
 
   const duplicateMutation = useMutation({
@@ -121,7 +122,7 @@ export function HeapsPanel() {
       toast.success('Heap duplicated', heap.name)
     },
     onError: (e: any) =>
-      toast.error('Failed to duplicate heap', e.message || 'Unknown error'),
+      toast.error('Failed to duplicate heap', formatApiError(e)),
   })
 
   // Drop handler: add the dragged photos to the target heap. Optimistically
@@ -142,7 +143,7 @@ export function HeapsPanel() {
       if (ctx?.previous) {
         queryClient.setQueryData(['heap-photo-ids', vars.heapId], ctx.previous)
       }
-      toast.error('Failed to add to heap', e.message || 'Unknown error')
+      toast.error('Failed to add to heap', formatApiError(e))
     },
     onSuccess: (data, vars) => {
       const heap = heaps.find((h) => h.id === vars.heapId)