feat(speaking): extend SessionConfig with per-mode prompt + greeting

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/types/englishSpeaking.ts

@@ -65,6 +65,12 @@ export interface TopicDiscussionConfig {
   evaluation: EvaluationSettings
   videoScene: {
     enabled: boolean
+    systemPrompt: string
+    greeting: string
+  }
+  nonVideoScene: {
+    systemPrompt: string
+    greeting: string
   }
 }
 
@@ -278,6 +284,8 @@ export interface SessionConfig {
   userId?: string | null
   mode?: 'normal' | 'video_scene'
   selectedVideo?: VideoMeta
+  systemPrompt?: string
+  greeting?: string
 }
 
 // 对话会话信息 (createSession 返回)