feat: enhance CoordinatorEmptyState and ThemeSelector with gradient utilities and improved layout

ccw/frontend/src/locales/en/theme.json

@@ -22,6 +22,9 @@
   "current": "Current theme: {name}",
   "hueValue": "Hue: {value}°",
   "preview": "Preview",
+  "preview.background": "Background",
+  "preview.surface": "Card",
+  "preview.accent": "Accent",
   "save": "Save Custom Theme",
   "reset": "Reset to Preset"
 }

ccw/frontend/src/locales/zh/theme.json

@@ -22,6 +22,9 @@
   "current": "当前主题: {name}",
   "hueValue": "色调: {value}°",
   "preview": "预览",
+  "preview.background": "背景",
+  "preview.surface": "卡片",
+  "preview.accent": "强调色",
   "save": "保存自定义主题",
   "reset": "重置为预设"
 }