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/layout/LeftSidebar.tsx

@@ -43,6 +43,7 @@ import {
   LIBRARY_STATS_QUERY_KEY,
 } from '../../hooks/useLibraryStatsQuery'
 import { registerUndoable } from '../../store/undoStore'
+import { formatApiError } from '../../lib/apiError'
 import type { Photo } from '../../types/photo'
 import { DeleteFolderDialog } from '../dialogs/DeleteFolderDialog'
 import { ShareDialog } from '../sharing/ShareDialog'
@@ -150,7 +151,7 @@ export function LeftSidebar() {
       queryClient.invalidateQueries({ queryKey: LIBRARY_STATS_QUERY_KEY })
     },
     onError: (e: any) =>
-      toast.error('Discard failed', e?.message || 'Unknown error'),
+      toast.error('Discard failed', formatApiError(e)),
   })
 
   // Bulk move mutation for the drag-onto-folder interaction. The mutation
@@ -223,7 +224,7 @@ export function LeftSidebar() {
       queryClient.invalidateQueries({ queryKey: ['folders'] })
     },
     onError: (e: any) =>
-      toast.error('Move failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+      toast.error('Move failed', formatApiError(e)),
   })
 
   // Bulk copy mutation — Alt-drag uses this instead of move.
@@ -245,7 +246,7 @@ export function LeftSidebar() {
       queryClient.invalidateQueries({ queryKey: ['folders'] })
     },
     onError: (e: any) =>
-      toast.error('Copy failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+      toast.error('Copy failed', formatApiError(e)),
   })
 
   // Reads the dragged ids out of a drop event payload.
@@ -314,7 +315,7 @@ export function LeftSidebar() {
       queryClient.invalidateQueries({ queryKey: ['photos'] })
     },
     onError: (e: any) =>
-      toast.error('Rename failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+      toast.error('Rename failed', formatApiError(e)),
   })
 
   const createFolderMutation = useMutation({
@@ -328,7 +329,7 @@ export function LeftSidebar() {
       setCreateDraft('')
     },
     onError: (e: any) =>
-      toast.error('Create failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+      toast.error('Create failed', formatApiError(e)),
   })
 
   // Toggle folder hide-from-views. Invalidates every query that could
@@ -354,7 +355,7 @@ export function LeftSidebar() {
       queryClient.invalidateQueries({ queryKey: ['tags'] })
     },
     onError: (e: any) =>
-      toast.error('Toggle failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+      toast.error('Toggle failed', formatApiError(e)),
   })
 
   const deleteFolderMutation = useMutation({
@@ -383,7 +384,7 @@ export function LeftSidebar() {
       setDeletingFolder(null)
     },
     onError: (e: any) =>
-      toast.error('Delete failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+      toast.error('Delete failed', formatApiError(e)),
   })
 
   const toggleExpanded = (id: string) => {