Modernize login, signup, and dashboard pages with contemporary design

Transformed key user-facing pages with modern UI patterns:

**Login Page (sessions/new.html.erb)**:
- Enhanced warning box with gradient background and backdrop blur
- Added arrow indicator to "Learn more" link
- Improved visual hierarchy with better icon sizing

**Signup Page (users/new.html.erb)**:
- Complete rewrite from Bootstrap 2 to Bootstrap 5
- Modern card-based layout matching login page aesthetic
- Icon-enhanced form inputs with proper labels
- Side-by-side first/last name fields
- Gradient info box with training environment notice
- Proper form validation attributes

**Dashboard Home (dashboard/home.html.erb)**:
- Replaced old .span12/.row-fluid with modern grid
- Clean card-based layout with shadow
- Icon-enhanced header and buttons
- Loading spinner states during chart transitions
- Active button state indicators for chart type toggle
- Turbolinks compatibility
- Improved accessibility with ARIA labels

All pages now feature:
- Bootstrap 5 components and utilities
- Bootstrap Icons integration
- Rounded corners and modern spacing
- Gradient accents and visual depth
- Smooth transitions and hover states

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/views/sessions/new.html.erb

@@ -64,13 +64,13 @@
       </div>
     <% end %>
 
-    <div class="mt-4 p-3 bg-warning bg-opacity-10 border border-warning rounded">
+    <div class="mt-4 p-3 rounded" style="background: linear-gradient(135deg, rgba(255, 193, 7, 0.1), rgba(255, 152, 0, 0.1)); border: 2px solid rgba(255, 193, 7, 0.3); backdrop-filter: blur(10px);">
       <div class="d-flex align-items-start">
-        <i class="bi bi-exclamation-triangle-fill text-warning me-2 mt-1"></i>
+        <i class="bi bi-exclamation-triangle-fill text-warning me-2 mt-1" style="font-size: 1.25rem;"></i>
         <div class="small">
-          <strong>Security Training Environment</strong><br>
+          <strong class="d-block mb-1">Security Training Environment</strong>
           This is an intentionally vulnerable application for educational purposes.
-          <a href="https://github.com/OWASP/railsgoat/wiki" target="_blank" class="text-decoration-none">Learn more</a>
+          <a href="https://github.com/OWASP/railsgoat/wiki" target="_blank" class="text-warning fw-semibold text-decoration-none">Learn more →</a>
         </div>
       </div>
     </div>