feat(mobile): 优化手机端浮动按钮样式与布局

- 浮动按钮改为半透明磨砂玻璃效果（白底 + backdrop-filter blur）
- 按钮改为上下竖排排列
- 按钮位置调整到屏幕中间偏上（35%）
- 手机端隐藏房间介绍和输入栏动作/字色/字号/禁音/分屏控件
- 抽屉改为从顶部向下滑入

public/css/chat.css

@@ -682,7 +682,7 @@ a:hover {
         top: 35%;
         right: 10px;
         display: flex;
-        flex-direction: row;
+        flex-direction: column;
         gap: 8px;
         z-index: 500;
     }
@@ -691,23 +691,26 @@ a:hover {
         width: 44px;
         height: 44px;
         border-radius: 50%;
-        border: none;
+        border: 1px solid rgba(51, 102, 153, 0.4);
-        background: var(--bg-header);
+        background: rgba(255, 255, 255, 0.82);
-        color: #fff;
+        backdrop-filter: blur(8px);
+        -webkit-backdrop-filter: blur(8px);
+        color: #336699;
         font-size: 18px;
         cursor: pointer;
-        box-shadow: 0 3px 10px rgba(0, 0, 0, 0.35);
+        box-shadow: 0 2px 10px rgba(0, 0, 0, 0.18);
         display: flex;
         align-items: center;
         justify-content: center;
-        transition: transform 0.15s, background 0.15s;
+        transition: transform 0.15s, background 0.15s, box-shadow 0.15s;
         user-select: none;
         -webkit-tap-highlight-color: transparent;
     }
 
     .mobile-fab:active {
         transform: scale(0.92);
-        background: var(--border-blue);
+        background: rgba(51, 102, 153, 0.15);
+        box-shadow: 0 1px 6px rgba(0, 0, 0, 0.2);
     }
 
     /* ── 抽屉遮罩 ── */