Fix accessibility violations: color contrast, aside nesting

- Improve text contrast from #666/#888/#999 to #555/#6b6b6b across site (WCAG AA)
- Fix aside nesting in all 10 blog articles (aside→div in related-articles)
- Fix aside nesting in quote sidebar
- Update CRO enhancement CSS/JS inline colors

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

assets/js/cro-enhancements.js

@@ -52,7 +52,7 @@
             '<div class="lead-magnet">' +
             '<div style="font-size:2.5em;margin-bottom:10px;">📊</div>' +
             '<strong>Free Data Project Checklist</strong>' +
-            '<p style="font-size:0.9em;margin:10px 0 0 0;color:#666;">' +
+            '<p style="font-size:0.9em;margin:10px 0 0 0;color:#555;">' +
             'The same checklist we use for enterprise projects</p>' +
             '</div>' +
             '<form id="exit-form" onsubmit="submitExitForm(event)">' +
@@ -137,7 +137,7 @@
     var notification = document.createElement("div");
     notification.id = "social-proof";
     notification.style.cssText = "position:fixed;bottom:80px;left:20px;background:#fff;padding:15px 20px;border-radius:8px;box-shadow:0 4px 20px rgba(0,0,0,0.15);z-index:9998;display:none;max-width:280px;font-size:14px;border-left:4px solid #00cc66;";
-    notification.innerHTML = '<div style="display:flex;align-items:center;gap:10px;"><span style="font-size:1.5em;">🔔</span><span id="social-proof-text"></span></div><span onclick="closeSocialProof()" style="position:absolute;top:5px;right:10px;cursor:pointer;color:#999;font-size:1.2em;">&times;</span>';
+    notification.innerHTML = '<div style="display:flex;align-items:center;gap:10px;"><span style="font-size:1.5em;">🔔</span><span id="social-proof-text"></span></div><span onclick="closeSocialProof()" style="position:absolute;top:5px;right:10px;cursor:pointer;color:#6b6b6b;font-size:1.2em;">&times;</span>';
     document.body.appendChild(notification);
     
     window.closeSocialProof = function() {