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.

harness/tht/session/store.py

@@ -151,13 +151,13 @@ def new_session_manifest(
 
 def pin_interaction_language(manifest: SessionManifest, workspace_language: str) -> bool:
     """Apply legacy compatibility once, while the repository holds its writer lock."""
-    from tht.session.language import validate_language_tag
+    from tht.session.language import question_interaction_language
 
     if manifest.interaction_language is not None:
         return False
     if manifest.status == "finalized" or manifest.archived:
         raise SessionError("Session is read-only (finalized or archived)")
-    manifest.interaction_language = validate_language_tag(workspace_language)
+    manifest.interaction_language = question_interaction_language(manifest.question, workspace_language)
     manifest.updated_at = datetime.now(UTC)
     manifest.updated_by = current_author()
     return True