feat(ux): welcome banner, dashboard auth fix, docs update

- WelcomeBanner.tsx: guest-only dismissible onboarding card on dashboard
  (localStorage pv_seen_welcome, Browse Bills CTA, X dismiss)
- useDashboard: add !!token to query key so login/logout triggers
  a fresh fetch without manual refresh
- ARCHITECTURE.md: WelcomeBanner component, auth-aware query keys,
  v0.6.1 feature history entry
- Roadmap: mark welcome banner items complete
- Add MVP planning notes (Phase 3-6 roadmap draft)

Co-Authored-By: Jack Levy

frontend/lib/hooks/useDashboard.ts

@@ -1,9 +1,11 @@
 import { useQuery } from "@tanstack/react-query";
 import { dashboardAPI } from "../api";
+import { useAuthStore } from "@/stores/authStore";
 
 export function useDashboard() {
+  const token = useAuthStore((s) => s.token);
   return useQuery({
-    queryKey: ["dashboard"],
+    queryKey: ["dashboard", !!token],
     queryFn: () => dashboardAPI.get(),
     staleTime: 2 * 60 * 1000,
     refetchInterval: 5 * 60 * 1000,