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/hooks/useDashboardLayout.ts

@@ -89,7 +89,16 @@ export function useDashboardLayout() {
     staleTime: 30_000,
   }) as { data: { layout: DashboardLayoutConfig | null; updatedAt: unknown } | null | undefined };
 
-  const saveMutation = trpc.user.saveDashboardLayout.useMutation();
+  const [saveStatus, setSaveStatus] = useState<"idle" | "saved">("idle");
+  const saveStatusTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  const saveMutation = trpc.user.saveDashboardLayout.useMutation({
+    onSuccess: () => {
+      setSaveStatus("saved");
+      if (saveStatusTimerRef.current) clearTimeout(saveStatusTimerRef.current);
+      saveStatusTimerRef.current = setTimeout(() => setSaveStatus("idle"), 2500);
+    },
+  });
 
   // Sync from DB on load (DB wins if it has data).
   useEffect(() => {
@@ -237,6 +246,7 @@ export function useDashboardLayout() {
   return {
     config,
     isHydrated,
+    saveStatus,
     addWidget,
     removeWidget,
     updateWidgetConfig,