feat(practice): run realtime practice on App-Plus through renderjs

App-Plus has no WebRTC or getUserMedia in the logic layer, so realtime
practice was H5-only and the native entry had to advertise a record-then-
transcribe downgrade.

Move the media/WebRTC engine into `realtime-browser-engine.js` and drive it
from `RealtimePracticeAppBridge.vue`, whose renderjs script runs inside the
system WebView where those APIs do exist. The logic layer keeps the
authenticated calls: SDP exchange and `search_knowledge` tool invocation stay
server-proxied, so the access token is never handed to the view layer. Both
sides talk over the renderjs bridge only in session payloads the server
already assembled.

Verified against a real App build (`app-renderjs.js` holds RTCPeerConnection
and getUserMedia; `app-service.js` holds the SDP endpoint, renderjs side has
zero hits). Microphone permission, live transcription, remote playback,
foreground/background switching and disconnect fallback still require Android
and iOS device verification.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/MOBILE_APP_DEVICE_ACCEPTANCE.md

@@ -14,7 +14,7 @@
 - [ ] 首次启动先显示原生隐私弹窗；两个协议链接可打开，拒绝后退出，同意后才进入登录。
 - [ ] 手机号登录、短信验证码错误、登录失效重登均符合预期。
 - [ ] 首次录音允许麦克风后，可完成转写；拒绝麦克风后不循环申请，能选择本地音频继续。
-- [ ] 对练入口在原生端明确降级为录音后转写，不出现不可用的浏览器 WebRTC 实时通话承诺。
+- [ ] 原生实时对练可完成麦克风授权、实时转写与远端语音播放；WebRTC/WebView 不可用或断线时明确提示并保留录音对练。
 - [ ] 图片、视频、文档和音频可选择并上传；受控附件下载后可打开，不泄露 OSS 地址。
 - [ ] 前后台切换、网络断开/恢复、窄屏和底部手势区无白屏、遮挡或重复提交。
 - [ ] 再次安装同版本后，桌面图标、启动图和 Android 导航栏不闪白。