Accessibility: fix WCAG AA colour contrast and cursor issues

- Darken #179e83 -> #148069 for backgrounds with white text (3.36->4.86:1)
- Darken #148a72 -> #148069 for green text on white (4.27->4.86:1)
- Darken trust-heading #6c757d -> #5c6370 on #f8f9fa (4.45->5.73:1)
- Switch #00cc66 bg elements (sticky CTA btn, case-study metric) to dark text
- Fix .btn:link color: inherit -> white (was inheriting body #333 instead of white)
- Add .btn:hover lift/shadow for bare .btn class
- cursor: default on body (stops I-beam appearing on non-interactive text)
- cro-enhancements: add role=complementary + aria-label to sticky CTA bar

assets/js/cro-enhancements.js

@@ -19,6 +19,8 @@
     // === STICKY CTA BAR ===
     var stickyBar = document.createElement("div");
     stickyBar.className = "sticky-cta-bar";
+    stickyBar.setAttribute("role", "complementary");
+    stickyBar.setAttribute("aria-label", "Contact prompt");
     stickyBar.innerHTML = '<p>Need expert help with your data project?</p>' +
         '<a href="/quote" class="btn" onclick="trackCTA(\'sticky_bar\')">Get Free Consultation</a>' +
         '<span class="close-bar" onclick="closeStickyBar()">&times;</span>';