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/cli/session_cmd.py

@@ -178,7 +178,8 @@ def list_cmd(
 def new_cmd(
     question: str = typer.Argument(..., help="La domanda in linguaggio naturale."),
     interaction_language: str = typer.Option(
-        None, "--interaction-language", help="BCP-47 interaction language; defaults to workspace language."
+        None, "--interaction-language",
+        help="BCP-47 fallback for ambiguous questions; otherwise use the question's language.",
     ),
     provider: str = typer.Option(None, "--provider", help="Provider LLM (es. zai, anthropic)."),
     model: str = typer.Option(None, "--model", help="Modello LLM (es. glm-5.2)."),
@@ -193,10 +194,11 @@ def new_cmd(
     from tht.session.store import _extract_name, new_session_manifest, render_question_md
 
     cfg = _load_config_or_exit(config)
-    from tht.session.language import validate_language_tag
+    from tht.session.language import question_interaction_language
 
     try:
-        language = validate_language_tag(
+        language = question_interaction_language(
+            question,
             interaction_language if interaction_language is not None else getattr(cfg, "language", "en")
         )
     except ValueError as exc:
@@ -209,7 +211,7 @@ def new_cmd(
     repository.create(manifest)
     repository.write_artifact(manifest.id, "question", render_question_md(question))
     if json_out:
-        typer.echo(json.dumps({"id": manifest.id}, ensure_ascii=False))
+        typer.echo(json.dumps({"id": manifest.id, "interaction_language": language}, ensure_ascii=False))
         return
     typer.secho(f"OK: sessione creata ({manifest.id})", fg=typer.colors.GREEN)
     typer.echo(manifest.id)
@@ -384,7 +386,7 @@ def ensure_interaction_language_cmd(
     json_out: bool = typer.Option(False, "--json", help="Emit pristine JSON."),
     config: Path = CONFIG_OPT,
 ) -> None:
-    """Pin a missing legacy language from the workspace; never override a saved value."""
+    """Pin a missing language from the question; never override a saved value."""
     cfg = _load_config_or_exit(config)
     manifest = ensure_session_language_or_exit(cfg, session_id)
     if json_out: