feat(frontend): /pricing route placeholder + UserResponse subscription fields

Two upstream additions to support private overlays implementing paid tiers
without forking client code:

- store/useAuthStore.ts: UserResponse extended with optional
  is_paid_subscriber, subscription_status, subscription_period_end. The
  backend now returns these in /api/auth/me; the persist middleware
  serialises them automatically.
- pages/PricingPlaceholder.tsx (NEW): the /pricing route. Renders a polite
  "this image is fully free" message for self-hosters plus a
  data-velxio-slot="pricing-page" target where private overlays can
  portal-inject a real pricing page.
- App.tsx: register the /pricing route after /about.

Self-hosted OSS image: /pricing shows the placeholder, no behavioural
change anywhere else. Production with a private overlay: /pricing shows
the overlay's full pricing UI.

Frontend build verified.

frontend/src/App.tsx

@@ -28,6 +28,7 @@ import { RaspberryPiSimulatorPage } from './pages/RaspberryPiSimulatorPage';
 import { Velxio2Page } from './pages/Velxio2Page';
 import { Velxio25Page } from './pages/Velxio25Page';
 import { AboutPage } from './pages/AboutPage';
+import { PricingPlaceholder } from './pages/PricingPlaceholder';
 import { useAuthStore } from './store/useAuthStore';
 import './App.css';
 
@@ -68,6 +69,8 @@ function App() {
         <Route path="/v2" element={<Velxio2Page />} />
         <Route path="/v2-5" element={<Velxio25Page />} />
         <Route path="/about" element={<AboutPage />} />
+        {/* Pricing — placeholder by default; private overlays portal-inject the real page */}
+        <Route path="/pricing" element={<PricingPlaceholder />} />
         {/* Canonical project URL by ID */}
         <Route path="/project/:id" element={<ProjectByIdPage />} />
         {/* Legacy slug route — redirects to /project/:id */}

frontend/src/pages/PricingPlaceholder.tsx

@@ -0,0 +1,69 @@
+/**
+ * PricingPlaceholder — the route at /pricing.
+ *
+ * In the open-source build this renders a minimal heading + slot div. It
+ * exists so that:
+ *  1. Private overlays can portal-inject a real PricingPage into the slot
+ *     (data-velxio-slot="pricing-page") without forking this file.
+ *  2. Self-hosters who navigate to /pricing get a polite "this image
+ *     doesn't sell anything" page rather than a 404.
+ *
+ * The route is registered in App.tsx. The slot pattern is the same as
+ * data-velxio-slot="user-menu" / "admin-tabs" / "admin-tab-content".
+ */
+
+import { useEffect } from 'react';
+import { AppHeader } from '../components/layout/AppHeader';
+
+export const PricingPlaceholder = () => {
+  useEffect(() => {
+    document.title = 'Pricing — Velxio';
+  }, []);
+
+  return (
+    <div style={{ minHeight: '100vh', background: '#1e1e1e', color: '#e8e8e8' }}>
+      <AppHeader />
+      <div data-velxio-slot="pricing-page">
+        {/* Default content — only visible if no overlay is mounted (i.e.
+            self-hosted OSS image without a private pricing overlay). */}
+        <main
+          style={{
+            maxWidth: 720,
+            margin: '60px auto',
+            padding: '0 24px',
+            fontFamily: '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif',
+            lineHeight: 1.6,
+          }}
+        >
+          <h1 style={{ marginTop: 0 }}>Pricing</h1>
+          <p>
+            This Velxio instance is self-hosted from the open-source image —
+            all features are free for everyone using it.
+          </p>
+          <p>
+            The hosted version at{' '}
+            <a
+              href="https://velxio.dev"
+              style={{ color: '#4fc3f7', textDecoration: 'none' }}
+            >
+              velxio.dev
+            </a>{' '}
+            offers an optional Pro tier that helps fund development.
+          </p>
+          <p style={{ color: '#888', fontSize: 13, marginTop: 32 }}>
+            Source code is{' '}
+            <a
+              href="https://github.com/davidmonterocrespo24/velxio"
+              style={{ color: '#888' }}
+            >
+              MIT-licensed on GitHub
+            </a>
+            .
+          </p>
+        </main>
+      </div>
+    </div>
+  );
+};
+
+export default PricingPlaceholder;

frontend/src/store/useAuthStore.ts

@@ -9,6 +9,12 @@ export interface UserResponse {
   avatar_url: string | null;
   is_admin: boolean;
   created_at: string;
+  // Subscription state — populated by deployments that wire an external
+  // billing system (e.g. velxio.dev). Self-hosted OSS images leave these
+  // at the safe defaults (no paid features unlock).
+  is_paid_subscriber?: boolean;
+  subscription_status?: string | null;
+  subscription_period_end?: string | null;
 }
 
 interface AuthState {