feat(widget): Widget 配置、loader 与右下角悬浮入口

生成可嵌入站点的 loader 与悬浮按钮，支持展开收起和移动端；附独立宿主测试页。

frontend/public/assets/images/logo_dark.svg

@@ -0,0 +1,20 @@
+<svg width="1200" height="1200" viewBox="0 0 120 120" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <!-- Chat bubble background with subtle outline -->
+  <path 
+    d="M95 20H25C22.2386 20 20 22.2386 20 25V75C20 77.7614 22.2386 80 25 80H45L55 95L65 80H95C97.7614 80 100 77.7614 100 75V25C100 22.2386 97.7614 20 95 20Z" 
+    fill="#f34611"
+    stroke="#EEEEEE"
+    stroke-width="1"
+  />
+  
+  <!-- Letter C -->
+  <path 
+    d="M75 40C75 40 65 35 60 35C50 35 45 42 45 50C45 58 50 65 60 65C65 65 75 60 75 60V52C75 52 68 55 63 55C57 55 53 53 53 50C53 47 57 45 63 45C68 45 75 48 75 48V40Z"
+    fill="#FFFFFF"
+  />
+  
+  <!-- Dots -->
+  <!-- <circle cx="45" cy="50" r="3" fill="#BFEEAA"/>
+  <circle cx="60" cy="50" r="3" fill="#BFEEAA"/>
+  <circle cx="75" cy="50" r="3" fill="#BFEEAA"/> -->
+</svg>