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/css/cro-enhancements.css

@@ -60,7 +60,7 @@
 }
 .sticky-cta-bar .btn {
     background: #00cc66;
-    color: #fff;
+    color: #1a1a1a;
     padding: 10px 24px;
     border-radius: 6px;
     text-decoration: none;
@@ -183,7 +183,7 @@
 }
 .case-study-inline .metric {
     background: #00cc66;
-    color: #fff;
+    color: #1a1a1a;
     padding: 15px 20px;
     border-radius: 8px;
     text-align: center;