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>

mobile-uni/src/services/realtime-browser-engine.d.ts

@@ -0,0 +1,30 @@
+export type RealtimeBrowserStatus =
+  | 'idle'
+  | 'connecting'
+  | 'connected'
+  | 'listening'
+  | 'speaking'
+  | 'calling-tool';
+
+export interface RealtimeBrowserCallbacks {
+  onStatus: (status: RealtimeBrowserStatus) => void;
+  onTraineeTranscript: (text: string) => void;
+  onCustomerTranscript: (text: string) => void;
+  onAudioPlaybackBlocked: (blocked: boolean) => void;
+  onToolCall: (callId: string, name: string, argsJson: string) => Promise<string>;
+  onTerminated: () => void;
+  onError: (message: string) => void;
+}
+
+export interface RealtimeBrowserEngine {
+  start: (sessionUpdate: Record<string, unknown>, sessionId?: string) => Promise<void>;
+  stop: () => void;
+  resumeAudio: () => Promise<boolean>;
+}
+
+export const isRealtimeBrowserSupported: () => boolean;
+
+export const createRealtimeBrowserEngine: (
+  callbacks: RealtimeBrowserCallbacks,
+  exchangeSdp: (offerSdp: string, sessionId?: string) => Promise<string>
+) => RealtimeBrowserEngine;