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/app/page.tsx

@@ -4,6 +4,7 @@ import { TrendingUp, BookOpen, Flame, RefreshCw } from "lucide-react";
 import Link from "next/link";
 import { useDashboard } from "@/lib/hooks/useDashboard";
 import { BillCard } from "@/components/shared/BillCard";
+import { WelcomeBanner } from "@/components/shared/WelcomeBanner";
 import { adminAPI } from "@/lib/api";
 import { useState } from "react";
 import { useAuthStore } from "@/stores/authStore";
@@ -40,6 +41,8 @@ export default function DashboardPage() {
         </button>
       </div>
 
+      <WelcomeBanner />
+
       {isLoading ? (
         <div className="text-center py-20 text-muted-foreground">Loading dashboard...</div>
       ) : (