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.

backend/src/routes/sessions.ts

@@ -509,12 +509,15 @@ export function sessionRoutes(
       // registry snapshot. The legacy fallback stays available for sessions created before
       // the browser-local preference migration.
       let id: string;
+      let sessionLanguage = language;
       try {
-        ({ id } = await runner.sessionNew({
+        const created = await runner.sessionNew({
           question: b.question, name: b.name, workspaceConfigPath,
           workspaceId, workspaceRevision, provider, model, thinking,
           interactionLanguage: language,
-        }));
+        });
+        id = created.id;
+        sessionLanguage = created.interaction_language ?? language;
         manifestPersisted = true;
         if (revisionLease) {
           await revisionLease.markPersisted().catch((error: unknown) => {
@@ -527,7 +530,7 @@ export function sessionRoutes(
       } catch { return storageFailure(reply); }
       const options = {
         provider, model, thinking,
-        interactionLanguage: language,
+        interactionLanguage: sessionLanguage,
         author: principal.displayName ?? principal.subject,
         principal,
         question: b.question,

backend/src/tht/tht-runner.ts

@@ -507,7 +507,7 @@ export class ThtRunner {
       if (v) a.push(f, v);
     }
     a.push("--json");
-    return this.json<{ id: string }>(a, o.workspaceConfigPath ?? o.workspace);
+    return this.json<{ id: string; interaction_language?: string }>(a, o.workspaceConfigPath ?? o.workspace);
   }
 
   /** Build and persist the deterministic F1 retrieval pack for a new session. */

backend/test/routes-sessions.test.ts

@@ -140,6 +140,33 @@ test("resume rejects browser interaction language overrides", async () => {
   } finally { await app.close(); }
 });
 
+test("new session starts Pi with the question language persisted by the harness", async () => {
+  let runtime: any;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      sessionNew: async () => ({ id: "english-question", interaction_language: "en" }),
+      searchPack: async () => {},
+    },
+    readiness: { ensure: async () => ({ ok: true }) },
+    mgr: {
+      get: () => undefined,
+      createFor: (_id: string, options: any) => {
+        runtime = options;
+        return { bridge: { onClientEvent: () => {} } };
+      },
+      configure: async () => {}, start: () => {},
+    },
+    getSettings: () => ({ workspace: "default" }),
+  });
+  try {
+    const response = await app.inject({ method: "POST", url: "/sessions", payload: {
+      question: "How many patients were admitted last year?", interactionLanguage: "it",
+    } });
+    expect(response.statusCode).toBe(200);
+    expect(runtime.interactionLanguage).toBe("en");
+  } finally { await app.close(); }
+});
+
 test("resume pins legacy interaction language using the resolved harness workspace", async () => {
   let pinnedWorkspace: string | undefined;
   let runtime: any;