fix(frontend): restore session context behavior in vertical slice

frontend/tests/vertical-slice.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, it, vi } from 'vitest';
-import { createVerticalSliceController } from '../src/spa/vertical-slice';
+import {
+  createVerticalSliceController,
+  type SessionContext,
+  type SessionContextStore
+} from '../src/spa/vertical-slice';
 import type { ApiClient } from '../src/api/client';
 
 function makeApiMock(overrides?: Partial<ApiClient>): ApiClient {
@@ -58,6 +62,16 @@ function makeApiMock(overrides?: Partial<ApiClient>): ApiClient {
   return { ...base, ...overrides };
 }
 
+function makeSessionContextStore(initial: SessionContext | null = null): SessionContextStore {
+  let value = initial;
+  return {
+    get: vi.fn(() => value),
+    set: vi.fn((next: SessionContext) => {
+      value = next;
+    })
+  };
+}
+
 describe('vertical slice controller: lobby -> join -> start round', () => {
   it('tracks loading and success state for join + start flow', async () => {
     const api = makeApiMock();
@@ -80,6 +94,36 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
     expect(postStart.startRoundState).toBe('success');
   });
 
+  it('persists session context after join and syncs normalized session code', async () => {
+    const api = makeApiMock();
+    const sessionContextStore = makeSessionContextStore();
+    const controller = createVerticalSliceController(api, sessionContextStore);
+
+    await controller.joinLobby('abcd12', 'Maja');
+
+    expect(sessionContextStore.set).toHaveBeenCalledWith({
+      sessionCode: 'ABCD12',
+      playerToken: 'token-1',
+      nickname: 'Maja'
+    });
+    expect(controller.getState().sessionCode).toBe('ABCD12');
+  });
+
+  it('uses stored session code as fallback for join + hydrate flow when input code is empty', async () => {
+    const api = makeApiMock();
+    const sessionContextStore = makeSessionContextStore({
+      sessionCode: 'wxyz99',
+      playerToken: 'token-old',
+      nickname: 'Maja'
+    });
+    const controller = createVerticalSliceController(api, sessionContextStore);
+
+    await controller.joinLobby('  ', 'Maja');
+
+    expect(api.joinSession).toHaveBeenCalledWith({ code: 'WXYZ99', nickname: 'Maja' });
+    expect(api.getSession).toHaveBeenCalledWith('ABCD12');
+  });
+
   it('surfaces a friendly error when join fails', async () => {
     const api = makeApiMock({
       joinSession: vi.fn().mockResolvedValue({