Increase mobile sidebar overlay opacity

Changed mobile overlay from 50% to 75% opacity (25% transparent)
to make the sidebar content more visible when open on mobile devices.

Updated rgba(0, 0, 0, 0.5) to rgba(0, 0, 0, 0.75)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

static/css/app.css

@@ -877,7 +877,7 @@ a:hover {
     display: none;
     position: fixed;
     inset: 0;
-    background-color: rgba(0, 0, 0, 0.5);
+    background-color: rgba(0, 0, 0, 0.75);
     z-index: 40;
 }