fix: clarify wechat existing account binding

frontend/src/views/auth/__tests__/WechatCallbackView.spec.ts

@@ -345,6 +345,35 @@ describe('WechatCallbackView', () => {
     expect(replaceMock.mock.calls[0]?.[0]).toContain('mode%3Dopen')
   })
 
+  it('binds directly to the current signed-in account during invitation flow', async () => {
+    exchangePendingOAuthCompletionMock.mockResolvedValue({
+      error: 'invitation_required',
+      redirect: '/usage',
+    })
+    getAuthTokenMock.mockReturnValue('current-auth-token')
+
+    const wrapper = mount(WechatCallbackView, {
+      global: {
+        stubs: {
+          AuthLayout: { template: '<div><slot /></div>' },
+          Icon: true,
+          RouterLink: { template: '<a><slot /></a>' },
+          transition: false,
+        },
+      },
+    })
+
+    await flushPromises()
+
+    expect(wrapper.find('[data-testid="existing-account-email"]').exists()).toBe(false)
+    await wrapper.get('[data-testid="existing-account-submit"]').trigger('click')
+
+    expect(prepareOAuthBindAccessTokenCookieMock).toHaveBeenCalledTimes(1)
+    expect(locationState.current.href).toContain('intent=bind_current_user')
+    expect(locationState.current.href).toContain('redirect=%2Fusage')
+    expect(locationState.current.href).toContain('mode=open')
+  })
+
   it('collects email for pending oauth account creation and submits adoption decisions', async () => {
     exchangePendingOAuthCompletionMock.mockResolvedValue({
       error: 'email_required',