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>

package.json

@@ -17,6 +17,7 @@
   },
   "devDependencies": {
     "@vitejs/plugin-vue": "^6.0.1",
+    "sass-embedded": "^1.93.3",
     "vite": "^7.2.4"
   }
 }