style: add button background and transition curve variable

- Add sass-embedded as dev dependency for SCSS compilation
- Add background color to buttons for better visibility
- Add --curve CSS variable for consistent easing transitions across components
- Rebuild compiled CSS files

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

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

public/assets/css/src/_buttons.scss

@@ -3,6 +3,7 @@ button {
 
   border: 1px solid var(--color-browngray-300);
   color: var(--color-browngray-300);
+  background-color: var(--color-panel-bg);
   border-radius: var(--border-radius);
   padding: 0.1rem 0.3rem;
 

public/assets/css/src/_variables.scss

@@ -7,4 +7,6 @@
   --border-radius: 0.2rem;
 
   --space-xs: 0.5rem;
+
+  --curve: cubic-bezier(0.86, 0, 0.07, 1);
 }