feat: make ignoring an event undoable

Ignoring made an event vanish with no way back except knowing the reveal
toggle existed — and the row you would click to undo is the row that just
disappeared.

Now: an undo appears the moment you ignore something, and revealed ignored
rows are labelled and carry a restore control where the done checkbox
normally sits, which is the most direct place for it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/client/components/Toast.tsx

@@ -0,0 +1,62 @@
+import { useEffect } from "react";
+
+/**
+ * A short-lived confirmation with an undo.
+ *
+ * Ignoring an event makes it vanish, which is exactly what was asked for and
+ * also the moment a mistake is least recoverable — the row you would click to
+ * undo is the row that just disappeared. The undo lives here instead.
+ */
+export function Toast({
+  message,
+  actionLabel,
+  onAction,
+  onDismiss,
+  ms = 7000,
+}: {
+  message: string;
+  actionLabel: string;
+  onAction: () => void;
+  onDismiss: () => void;
+  ms?: number;
+}) {
+  useEffect(() => {
+    const id = setTimeout(onDismiss, ms);
+    return () => clearTimeout(id);
+  }, [onDismiss, ms, message]);
+
+  return (
+    <div
+      role="status"
+      aria-live="polite"
+      className="pointer-events-none fixed inset-x-0 bottom-4 z-50 flex justify-center px-4"
+    >
+      <div className="pointer-events-auto flex max-w-md items-center gap-3 rounded-xl border border-hairline bg-raised px-4 py-3 shadow-lg">
+        <p className="min-w-0 flex-1 truncate text-xs text-muted">{message}</p>
+        <button
+          type="button"
+          onClick={onAction}
+          className="shrink-0 rounded-md px-2 py-1 text-xs font-semibold text-near transition-colors duration-150 hover:text-ink"
+        >
+          {actionLabel}
+        </button>
+        <button
+          type="button"
+          onClick={onDismiss}
+          aria-label="Dismiss"
+          className="shrink-0 text-faint transition-colors duration-150 hover:text-muted"
+        >
+          <svg viewBox="0 0 16 16" className="size-3.5" aria-hidden>
+            <path
+              d="M4 4l8 8M12 4l-8 8"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="2"
+              strokeLinecap="round"
+            />
+          </svg>
+        </button>
+      </div>
+    </div>
+  );
+}