refactor: migrate landing page to Design System v4

Replace glass/gradient/blur effects with flat dark theme:
- Solid bg-[#14161d] cards instead of glass surfaces
- Solid #22d3ee buttons instead of gradient CTAs
- #67e8f9 text instead of gradient text
- Flat nav background (no backdrop-filter blur)
- Subtle radial-gradient hero glow instead of animated pulses
- Featured pricing card uses border-[#22d3ee]
- Remove all transition: all per lesson 81
- Update color tokens: #e2e5eb foreground, #848b9b muted, #1e2130 borders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/LandingPage.tsx

@@ -194,7 +194,7 @@ export default function LandingPage() {
               <div className="landing-preview-body">
                 <div className="landing-preview-sidebar">
                   <div className="landing-preview-sidebar-item active">
-                    <div className="dot" style={{ background: 'var(--cyan-400)' }} />
+                    <div className="dot" style={{ background: '#22d3ee' }} />
                     Flow Editor
                   </div>
                   <div className="landing-preview-sidebar-item">
@@ -311,7 +311,7 @@ export default function LandingPage() {
                         <span className="text">Is the user on VPN?</span>
                       </div>
                       <div className="landing-mock-chat-line">
-                        <span className="label" style={{ color: 'var(--text-secondary)' }}>Engineer:</span>
+                        <span className="label" style={{ color: '#848b9b' }}>Engineer:</span>
                         <span className="text">Yes, Cisco AnyConnect</span>
                       </div>
                       <div className="landing-mock-chat-line">