Fix white button text colour in dark mode

btn-white appears inside section-dark sections which stay dark;
pin its text to #0A0A0A so it doesn't inherit the flipped --ink token.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

style.css

@@ -1073,6 +1073,8 @@ html.dark .btn-outline {
 
 html.dark .btn-outline:hover { border-color: #F0F0ED; }
 
+html.dark .btn-white { color: #0A0A0A; }
+
 html.dark .industry-cell { background: #0F0F0E; }
 html.dark .section-off .industry-cell { background: #161615; }