Improve cart and dark mode UX

- Clear cart when customer logs out
- Hide cart bag button for non-authenticated users (only show for logged-in customers)
- Fix dark mode text visibility in all form inputs (input/textarea/select elements)

Addresses user request: cart should clear on logout and only be accessible to logged-in customers.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/components/LogoutOnUnload.tsx

@@ -1,14 +1,19 @@
 'use client';
 
 import { useEffect } from 'react';
+import { useCart } from '@/lib/cartStore';
 
 /**
  * Logs out the user when they close/leave the page
  * Works for both admin and customer sessions
  */
 export default function LogoutOnUnload() {
+  const clearCart = useCart((s) => s.clear);
+
   useEffect(() => {
     const handleUnload = () => {
+      // Clear cart on logout
+      clearCart();
       // Use sendBeacon to reliably send logout even when page is closing
       // This doesn't wait for a response, which is fine for logout
       navigator.sendBeacon('/api/logout');
@@ -21,7 +26,7 @@ export default function LogoutOnUnload() {
       window.removeEventListener('beforeunload', handleUnload);
       window.removeEventListener('unload', handleUnload);
     };
-  }, []);
+  }, [clearCart]);
 
   return null;
 }