style: hide the default white scrollbar corner

::-webkit-scrollbar-corner was unstyled so the browser filled the
intersection of horizontal + vertical scrollbars with its default
color, which read as a ~10×10 white square in the dark theme.
Transparent matches the already-transparent track.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/styles/global.css

@@ -146,6 +146,10 @@ textarea:focus {
   background: var(--border-strong);
 }
 
+::-webkit-scrollbar-corner {
+  background: transparent;
+}
+
 ::selection {
   background: var(--accent-soft-strong);
   color: var(--text);