Delay item disappearance after check-off; add setting in Settings

Items on the lists page and dashboard widget now show as crossed-out for
a configurable duration (default 3s) before vanishing. Unchecking within
the window cancels the removal. The delay is stored in localStorage and
configurable via Settings → Lists (options: 1s / 3s / 5s / 10s / 30s).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/completion-delay-setting.tsx

@@ -0,0 +1,30 @@
+"use client";
+
+import { DELAY_OPTIONS, useCompletionDelay } from "@/hooks/use-completion-delay";
+
+export function CompletionDelaySetting() {
+  const { delay, setDelay } = useCompletionDelay();
+
+  return (
+    <div className="flex items-center justify-between gap-4">
+      <div>
+        <p className="text-sm font-medium">Completion delay</p>
+        <p className="text-xs text-muted-foreground">
+          How long a checked-off item stays visible before disappearing from list cards and the
+          dashboard.
+        </p>
+      </div>
+      <select
+        value={delay}
+        onChange={(e) => setDelay(Number(e.target.value))}
+        className="rounded-md border border-input bg-background px-3 py-1.5 text-sm shadow-sm focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring"
+      >
+        {DELAY_OPTIONS.map((opt) => (
+          <option key={opt.value} value={opt.value}>
+            {opt.label}
+          </option>
+        ))}
+      </select>
+    </div>
+  );
+}