feat: add gradient effects settings and enhance theme customization options

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

@@ -26,5 +26,13 @@
   "preview.surface": "Card",
   "preview.accent": "Accent",
   "save": "Save Custom Theme",
-  "reset": "Reset to Preset"
+  "reset": "Reset to Preset",
+  "gradient": {
+    "title": "Gradient Effects",
+    "off": "Off",
+    "standard": "Standard",
+    "enhanced": "Enhanced",
+    "hoverGlow": "Enable hover glow effects",
+    "bgAnimation": "Enable background gradient animation"
+  }
 }

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

@@ -26,5 +26,13 @@
   "preview.surface": "卡片",
   "preview.accent": "强调色",
   "save": "保存自定义主题",
-  "reset": "重置为预设"
+  "reset": "重置为预设",
+  "gradient": {
+    "title": "渐变效果",
+    "off": "关闭",
+    "standard": "标准",
+    "enhanced": "增强",
+    "hoverGlow": "启用悬停光晕效果",
+    "bgAnimation": "启用背景渐变动画"
+  }
 }