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/dialogs/ConfirmDialog.tsx

@@ -0,0 +1,75 @@
+import { useEffect } from 'react'
+import clsx from 'clsx'
+
+interface ConfirmDialogProps {
+  isOpen: boolean
+  title: string
+  message: React.ReactNode
+  confirmLabel?: string
+  cancelLabel?: string
+  /** When true, the confirm button uses the destructive accent. */
+  destructive?: boolean
+  onConfirm: () => void
+  onClose: () => void
+}
+
+/**
+ * Tiny modal-confirmation dialog. Mirrors the AddSourceFolderDialog overlay
+ * pattern (custom fixed inset-0 backdrop, no shadcn Dialog dep). Esc closes.
+ */
+export function ConfirmDialog({
+  isOpen,
+  title,
+  message,
+  confirmLabel = 'Confirm',
+  cancelLabel = 'Cancel',
+  destructive = false,
+  onConfirm,
+  onClose,
+}: ConfirmDialogProps) {
+  // Esc to close.
+  useEffect(() => {
+    if (!isOpen) return
+    const handler = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') onClose()
+    }
+    window.addEventListener('keydown', handler)
+    return () => window.removeEventListener('keydown', handler)
+  }, [isOpen, onClose])
+
+  if (!isOpen) return null
+
+  return (
+    <div className="fixed inset-0 z-50">
+      <div
+        className="absolute inset-0 bg-black/60 backdrop-blur-sm"
+        onClick={onClose}
+      />
+      <div className="absolute left-1/2 top-1/2 -translate-x-1/2 -translate-y-1/2">
+        <div className="relative z-10 w-96 rounded-lg border border-border bg-surface p-5 shadow-2xl">
+          <h2 className="mb-2 text-base font-semibold text-text">{title}</h2>
+          <div className="mb-4 text-sm text-text-muted">{message}</div>
+          <div className="flex justify-end gap-2">
+            <button
+              onClick={onClose}
+              className="rounded border border-border px-3 py-1.5 text-sm text-text hover:bg-surface-2"
+            >
+              {cancelLabel}
+            </button>
+            <button
+              onClick={onConfirm}
+              className={clsx(
+                'rounded px-3 py-1.5 text-sm font-medium text-white',
+                destructive
+                  ? 'bg-reject hover:bg-reject/80'
+                  : 'bg-primary hover:bg-primary/80'
+              )}
+            >
+              {confirmLabel}
+            </button>
+          </div>
+        </div>
+      </div>
+    </div>
+  )
+}