Fix new-question landing and question-language HITL

Reset the activity panel when starting a new question so the landing navigation is restored. Detect and persist the original question language, pass it through runtime and widget descriptors, and scope HITL controls to that language.

Validated with gate, session, backend and frontend tests, TypeScript checks, Ruff and strict docs build. Rebuilt and restarted local core/frontend; both healthy and serving HTTP successfully.

frontend/src/api/types.ts

@@ -31,6 +31,7 @@ export interface SchemaTable {
 
 export interface WidgetDescriptor {
   id: string;
+  interaction_language?: string;
   schema_version?: number;
   session_id?: string;
   phase?: string;