fix(app): show ui switcher banner in demo mode

render_banner() was incorrectly guarded by 'if not IS_DEMO' — the spec
says the banner is open to all demo visitors. render_banner() already
handles its own eligibility check internally (_DEMO_MODE or can_use).

app/app.py

@@ -203,12 +203,11 @@ if IS_DEMO:
     from app.components.demo_toolbar import render_demo_toolbar
     render_demo_toolbar()
 
-# ── UI switcher banner (non-demo, paid tier) ────────────────────────────────
-if not IS_DEMO:
-    try:
+# ── UI switcher banner (paid tier; or all visitors in demo mode) ─────────────
+try:
     from app.components.ui_switcher import render_banner
     render_banner()
-    except Exception:
+except Exception:
     pass  # never crash the app over the banner
 
 pg.run()