修复视频通话振铃阶段本地小窗预览提前可见且可拖动

本地小窗预览（.call-party.self）此前只由 audioOnly/minimized 控制显隐，
没有校验连接是否真正建立（this.state.remoteStream），导致视频通话振铃
阶段（对方尚未接听、只有本地摄像头画面）小窗就已经以 display:block 可见，
且 dragStart 里对应的分支同样没有这层校验，振铃阶段就能拖动这个本应还
不存在的预览框。

render() 里 .call-party.self 的 disabled 属性补上 !connected 判断，
dragStart 里进入 'self' 拖动分支的条件补上 this.state.remoteStream 校验，
两处保持一致：只有远端媒体真正到达（连接建立）后小窗才出现并可拖动。

验证：用 canvas.captureStream()/AudioContext 构造的合法 MediaStreamTrack
配合 monkey-patch getUserMedia，绕开已知后端通话邀请 400 缺陷直接驱动
CallPanel 组件状态。振铃阶段（waitingForPeer=true, remoteStream 未设置）
下 .call-party.self 计算样式 display:none、无 [disabled] 属性对应的
cursor:grab；调用 handleTrackEvent 模拟远端媒体到达后，display:block、
cursor:grab，且用真实鼠标拖拽（非合成事件）验证了拖动/边界收紧都正常。

src/widgets/call-panel.jsx

@@ -163,11 +163,15 @@ class CallPanel extends React.PureComponent {
       // Fullscreen connected video call: let the user drag their own small self-view preview
       // out of the way instead of it being stuck wherever it defaults to (spec 8.9 "双方视频
       // 通话中": "小窗应支持拖动且不能遮挡关键按钮"). Only relevant when the box actually exists
-      // (base.css hides it entirely for audio-only calls / while minimized via
+      // (base.css hides it entirely for audio-only calls / while minimized, and now also while
+      // still ringing -- see the render()'s `disabled` prop just below -- via
       // .call-party.self[disabled]) and only when the drag actually starts on that box, not on
-      // the peer's video/avatar card behind it.
-      if (!this.props.minimized && !this.state.audioOnly && this.selfBoxRef.current &&
-          event.target.closest('.call-party.self')) {
+      // the peer's video/avatar card behind it. The `this.state.remoteStream` check mirrors
+      // render()'s own `connected` gate: without it, a pointerdown during the pre-connect ringing
+      // phase (box still hidden by CSS, but not yet [disabled] before this fix) would already set
+      // dragTarget='self' and start moving an invisible box.
+      if (!this.props.minimized && !this.state.audioOnly && this.state.remoteStream &&
+          this.selfBoxRef.current && event.target.closest('.call-party.self')) {
         event.stopPropagation();
         const el = this.selfBoxRef.current;
         // Pointer events are captured on the outer container (same element the listeners below
@@ -956,7 +960,7 @@ class CallPanel extends React.PureComponent {
           <div className="call-duration">{formatElapsedTime(this.state.elapsedSeconds)}</div>}
         <div id="video-container-panel" className="draggable">
           <div className="call-party self" ref={this.selfBoxRef}
-            disabled={this.state.audioOnly || this.props.minimized}>
+            disabled={this.state.audioOnly || this.props.minimized || !connected}>
             <video ref={this.localRef} autoPlay muted playsInline />
             <div className="caller-name inactive">
               <FormattedMessage id="calls_you_label"