feat: Improve UI layout and user experience

- Restructure index page to show welcome section only when IMAP is not configured
- Move stats and search sections to display only after IMAP setup
- Update header text color to neutral for better contrast
- Enhance welcome section with larger button and privacy assurance
- Add shield icon to emphasize data privacy protection

app/templates/partials/header.html

@@ -1,6 +1,6 @@
 <header class="bg-base-100 border-b border-base-300 p-4 flex justify-between  shadow-sm fixed top-0 left-0 right-0 z-20">
     <div>
-        <h1 class="text-2xl font-bold text-primary flex items-center">
+        <h1 class="text-2xl font-bold text-neutral flex items-center">
             <i class="fas fa-envelope mr-2"></i>
             Email Organizer
         </h1>