feat: discard view with restore and empty actions

Adds the destructive-action loop the discard concept needed:

- Click "Discarded" in the left sidebar → activates the discarded
  filter; the timeline reloads showing discarded photos.
- DiscardActionBar appears at the top of the timeline only when the
  discarded filter is active. Shows the count, a Restore button (when
  photos are selected), and an Empty discard pile button.
- Empty action goes through a ConfirmDialog (new tiny reusable modal,
  same overlay pattern as AddSourceFolderDialog).
- Restore goes through POST /api/v1/discard/restore.
- DELETE /api/v1/discard/empty now actually os.unlink()s the files
  from disk in addition to removing the DB rows. Per-file failures
  are logged and reported in the response so a single permission
  error doesn't abort the batch.

Other library nodes wired in passing:
- "All Photos"   → clearAll()
- "Rated"        → setRatingMin(1)
- "Flagged"      → setFlag('picked')
- "Discarded"    → setFlag('discarded')
- "By Date"      left unwired (needs a date-grouping UI)

Single-photo restore via the U keyboard shortcut already worked from
an earlier round, no change needed.

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

frontend/src/components/discard/DiscardActionBar.tsx

@@ -0,0 +1,112 @@
+import { useState } from 'react'
+import { RotateCcw, Trash2 } from 'lucide-react'
+import { useMutation, useQueryClient } from '@tanstack/react-query'
+import { usePhotoStore } from '../../store/photoStore'
+import { useFilterStore } from '../../store/filterStore'
+import { usePhotosQuery } from '../../hooks/usePhotosQuery'
+import { discard as discardApi } from '../../services/api'
+import { toast } from '../ToastContainer'
+import { ConfirmDialog } from '../dialogs/ConfirmDialog'
+
+/**
+ * Top-of-timeline bar visible only when the discarded filter is active.
+ * Shows a count, lets the user restore the current selection, and lets them
+ * permanently empty the discard pile (with confirmation).
+ */
+export function DiscardActionBar() {
+  const flag = useFilterStore((s) => s.flag)
+  const selectedPhotos = usePhotoStore((s) => s.selectedPhotos)
+  const clearSelection = usePhotoStore((s) => s.clearSelection)
+  const queryClient = useQueryClient()
+  const { data: photos = [] } = usePhotosQuery()
+
+  const [confirmOpen, setConfirmOpen] = useState(false)
+
+  const restoreMutation = useMutation({
+    mutationFn: (ids: string[]) => discardApi.restore(ids),
+    onSuccess: (_, ids) => {
+      toast.success('Restored', `${ids.length} photo${ids.length > 1 ? 's' : ''} restored`)
+      clearSelection()
+      queryClient.invalidateQueries({ queryKey: ['photos'] })
+    },
+    onError: (e: any) => toast.error('Restore failed', e.message || 'Unknown error'),
+  })
+
+  const emptyMutation = useMutation({
+    mutationFn: () => discardApi.empty(),
+    onSuccess: (data: any) => {
+      const count = data?.deleted ?? 0
+      const errors = data?.file_errors ?? 0
+      if (errors > 0) {
+        toast.error(
+          `Emptied with ${errors} error${errors > 1 ? 's' : ''}`,
+          `${count} record${count > 1 ? 's' : ''} deleted; some files could not be removed`
+        )
+      } else {
+        toast.success('Discard pile emptied', `${count} photo${count > 1 ? 's' : ''} permanently deleted`)
+      }
+      clearSelection()
+      queryClient.invalidateQueries({ queryKey: ['photos'] })
+      setConfirmOpen(false)
+    },
+    onError: (e: any) => toast.error('Empty failed', e.message || 'Unknown error'),
+  })
+
+  if (flag !== 'discarded') return null
+
+  const total = photos.length
+  const selected = selectedPhotos.length
+
+  return (
+    <>
+      <div className="flex items-center justify-between gap-3 border-b border-border bg-reject/10 px-4 py-2 text-sm">
+        <div className="flex items-center gap-2 text-text">
+          <Trash2 className="h-4 w-4 text-reject" />
+          <span className="font-medium">Discarded</span>
+          <span className="text-text-muted">
+            {total} photo{total === 1 ? '' : 's'}
+          </span>
+        </div>
+
+        <div className="flex items-center gap-2">
+          {selected > 0 && (
+            <button
+              onClick={() => restoreMutation.mutate(selectedPhotos)}
+              disabled={restoreMutation.isPending}
+              className="flex items-center gap-1.5 rounded bg-surface-2 px-3 py-1 text-text hover:bg-surface-offset disabled:opacity-50"
+              title="Restore selected (U)"
+            >
+              <RotateCcw className="h-3.5 w-3.5" />
+              Restore {selected}
+            </button>
+          )}
+          <button
+            onClick={() => setConfirmOpen(true)}
+            disabled={total === 0 || emptyMutation.isPending}
+            className="flex items-center gap-1.5 rounded bg-reject/20 px-3 py-1 text-reject hover:bg-reject/30 disabled:opacity-50"
+            title="Permanently delete all discarded photos and files"
+          >
+            <Trash2 className="h-3.5 w-3.5" />
+            Empty discard pile
+          </button>
+        </div>
+      </div>
+
+      <ConfirmDialog
+        isOpen={confirmOpen}
+        title="Empty discard pile?"
+        message={
+          <>
+            This will <strong className="text-text">permanently delete</strong>{' '}
+            {total} photo{total === 1 ? '' : 's'} and remove the file
+            {total === 1 ? '' : 's'} from disk. This cannot be undone.
+          </>
+        }
+        confirmLabel="Empty pile"
+        destructive
+        onConfirm={() => emptyMutation.mutate()}
+        onClose={() => setConfirmOpen(false)}
+      />
+    </>
+  )
+}