feat(ux): Sprint 1 — quick wins: EmptyState, DateRangePresets, debounce, save feedback, scenarios nav

- EmptyState shared component; replace AllocationsClient inline empty state
- DateRangePresets (this month/quarter/3 months/year) integrated into AllocationModal
- Debounce conflict-check inputs in AllocationModal (400ms) using existing useDebounce
- Dashboard layout save feedback via SuccessToast after DB write completes
- Scenarios nav item in Planning sidebar + /scenarios list page

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

apps/web/src/components/ui/EmptyState.tsx

@@ -0,0 +1,36 @@
+import type { ReactNode } from "react";
+
+interface EmptyStateProps {
+  icon?: ReactNode;
+  title: string;
+  detail?: string;
+  action?: {
+    label: string;
+    onClick: () => void;
+  };
+  testId?: string;
+}
+
+export function EmptyState({ icon, title, detail, action, testId }: EmptyStateProps) {
+  return (
+    <div
+      data-testid={testId}
+      className="flex flex-col items-center gap-2 py-12 text-center text-sm text-gray-500 dark:text-gray-400"
+    >
+      {icon && (
+        <div className="mb-1 text-gray-300 dark:text-gray-600">{icon}</div>
+      )}
+      <p className="font-medium text-gray-700 dark:text-gray-200">{title}</p>
+      {detail && <p>{detail}</p>}
+      {action && (
+        <button
+          type="button"
+          onClick={action.onClick}
+          className="mt-1 rounded-lg border border-gray-300 px-3 py-1.5 text-xs font-medium text-gray-700 transition hover:border-gray-400 hover:bg-gray-50 dark:border-gray-600 dark:text-gray-200 dark:hover:bg-gray-800"
+        >
+          {action.label}
+        </button>
+      )}
+    </div>
+  );
+}