fix(phase2e-2): 会议内聊天面板 UI 布局优化

问题修复：
- 原遮罩式抽屉覆盖底部 toolbar，导致按钮不可点击
- 发送按钮过小不显眼
- 聊天面板打开时视频区被遮挡出现大片空白

改进：
- room.vue 主区改为 flex-row 布局：视频列 + 聊天列并排，聊天打开时视频自动收窄
- ChatPanel 去除全屏遮罩，作为侧边栏嵌入 chat-col，保留过渡动画
- 发送按钮增加纸飞机图标并采用渐变背景，hover / active / disabled 状态清晰
- msg-list 使用 flex:1 + min-height:0 + height:0 修复 scroll-view 高度撑破问题，保证输入区始终固定在底部
- 小屏（<750px）自动回退为全屏浮层，避免挤压视频

Made-with: Cursor

frontend/src/pages/meeting/room.vue

@@ -34,22 +34,38 @@
       </view>
     </view>
 
-    <!-- 视频网格 -->
+    <!-- 主区：左视频 + 右侧聊天面板（仅 chatVisible 时侧栏展开） -->
     <view class="body">
-      <VideoGrid :tiles="tiles">
-        <template #tile="{ tile }">
-          <VideoTile
-            :user-id="tile.userId"
-            :name="tile.name"
-            :is-local="tile.isLocal"
-            :is-host="tile.isHost"
-            :audio-enabled="tile.audioEnabled"
-            :video-enabled="tile.videoEnabled"
-            :audio-track="tile.audioTrack"
-            :video-track="tile.videoTrack"
-          />
-        </template>
-      </VideoGrid>
+      <view class="video-col">
+        <VideoGrid :tiles="tiles">
+          <template #tile="{ tile }">
+            <VideoTile
+              :user-id="tile.userId"
+              :name="tile.name"
+              :is-local="tile.isLocal"
+              :is-host="tile.isHost"
+              :audio-enabled="tile.audioEnabled"
+              :video-enabled="tile.videoEnabled"
+              :audio-track="tile.audioTrack"
+              :video-track="tile.videoTrack"
+            />
+          </template>
+        </VideoGrid>
+      </view>
+      <view class="chat-col" :class="{ open: chatVisible }">
+        <ChatPanel
+          :visible="chatVisible"
+          :messages="chatMessages"
+          :current-user-id="myUserId"
+          :display-name-map="displayNameMap"
+          :has-more="chatHasMore"
+          :loading-more="chatLoadingMore"
+          @close="closeChat"
+          @send="onChatSend"
+          @load-more="onChatLoadMore"
+          @request-initial-load="onChatRequestInitialLoad"
+        />
+      </view>
     </view>
 
     <!-- 底部工具条 -->
@@ -92,20 +108,6 @@
       @close="inviteVisible = false"
     />
 
-    <!-- 聊天面板 -->
-    <ChatPanel
-      :visible="chatVisible"
-      :messages="chatMessages"
-      :current-user-id="myUserId"
-      :display-name-map="displayNameMap"
-      :has-more="chatHasMore"
-      :loading-more="chatLoadingMore"
-      @close="closeChat"
-      @send="onChatSend"
-      @load-more="onChatLoadMore"
-      @request-initial-load="onChatRequestInitialLoad"
-    />
-
     <!-- 离会确认弹窗 -->
     <view v-if="leaveDialogVisible" class="leave-mask" @click.self="leaveDialogVisible = false">
       <view class="leave-modal">
@@ -633,6 +635,31 @@ onUnload(() => {
   min-height: 0;
   background: #0B1220;
   overflow: hidden;
+  display: flex;
+  flex-direction: row;
+}
+.video-col {
+  flex: 1;
+  min-width: 0;
+  min-height: 0;
+  display: flex;
+  overflow: hidden;
+}
+.chat-col {
+  width: 0;
+  min-height: 0;
+  flex-shrink: 0;
+  overflow: hidden;
+  transition: width 0.24s ease;
+  border-left: 1rpx solid rgba(255, 255, 255, 0.06);
+  background: #1F2937;
+}
+.chat-col.open { width: 640rpx; }
+
+@media (max-width: 750px) {
+  /* 小屏幕：聊天面板改为全屏浮层，避免挤压视频 */
+  .chat-col { position: fixed; top: 0; right: 0; bottom: 0; z-index: 205; }
+  .chat-col.open { width: 100vw; }
 }
 
 /* 离会弹窗 */