feat(backgrounds): add snowfall background

src/locales/zh.json

@@ -1037,6 +1037,8 @@
       "rippleDesc": "涟漪波纹效果",
       "fireflies": "萤火虫",
       "firefliesDesc": "漂浮闪烁的发光点",
+      "snowfall": "降雪",
+      "snowfallDesc": "随风飘落的雪花",
       "geminiEffect": "Gemini效果",
       "geminiEffectDesc": "类似Google Gemini的SVG效果",
       "speed": "速度",
@@ -1058,6 +1060,7 @@
       "explosionColor": "爆炸颜色",
       "multicolor": "多彩模式",
       "lineColor": "线条颜色",
+      "wind": "风力",
       "bgColor": "背景颜色",
       "fillColor": "填充颜色",
       "color1": "颜色1",