fix(profile): stabilize binding compatibility and frontend checks

frontend/src/views/auth/LinuxDoCallbackView.vue

@@ -456,7 +456,14 @@ function resolvePendingAccountAction(
   if (raw === 'email_required' || raw === 'create_account_required' || raw === 'create_account') {
     return 'create_account'
   }
-  if (raw === 'bind_login_required' || raw === 'bind_login') {
+  if (
+    raw === 'bind_login_required' ||
+    raw === 'bind_login' ||
+    raw === 'existing_account' ||
+    raw === 'existing_account_required' ||
+    raw === 'existing_account_binding_required' ||
+    raw === 'adopt_existing_user_by_email'
+  ) {
     return 'bind_login'
   }
   return 'none'

frontend/src/views/auth/WechatCallbackView.vue

@@ -613,8 +613,12 @@ async function handleBindCurrentAccount() {
     return
   }
 
-  await prepareOAuthBindAccessTokenCookie()
-  window.location.href = startURL
+  try {
+    await prepareOAuthBindAccessTokenCookie()
+    window.location.href = startURL
+  } catch (e: unknown) {
+    errorMessage.value = getRequestErrorMessage(e, t('auth.loginFailed'))
+  }
 }
 
 async function handleExistingAccountBinding() {

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

@@ -336,6 +336,33 @@ describe('LinuxDoCallbackView', () => {
     )
   })
 
+  it('keeps rendering bind-login UI for legacy pending bind responses instead of treating them as success', async () => {
+    exchangePendingOAuthCompletion.mockResolvedValue({
+      error: 'adopt_existing_user_by_email',
+      redirect: '/profile/security',
+      email: 'existing@example.com'
+    })
+
+    const wrapper = mount(LinuxDoCallbackView, {
+      global: {
+        stubs: {
+          AuthLayout: { template: '<div><slot /></div>' },
+          Icon: true,
+          RouterLink: { template: '<a><slot /></a>' },
+          transition: false
+        }
+      }
+    })
+
+    await flushPromises()
+
+    expect(showSuccess).not.toHaveBeenCalled()
+    expect(replace).not.toHaveBeenCalled()
+    expect((wrapper.get('[data-testid="linuxdo-bind-login-email"]').element as HTMLInputElement).value).toBe(
+      'existing@example.com'
+    )
+  })
+
   it('persists a pending auth session when the oauth flow still needs account creation', async () => {
     exchangePendingOAuthCompletion.mockResolvedValue({
       error: 'email_required',

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

@@ -621,6 +621,34 @@ describe('WechatCallbackView', () => {
     expect(locationState.current.href).toContain('mode=open')
   })
 
+  it('shows an error and stays on the page when preparing bind-token for the current account fails', async () => {
+    exchangePendingOAuthCompletionMock.mockResolvedValue({
+      error: 'invitation_required',
+      redirect: '/usage',
+    })
+    getAuthTokenMock.mockReturnValue('current-auth-token')
+    prepareOAuthBindAccessTokenCookieMock.mockRejectedValue(new Error('bind token failed'))
+
+    const wrapper = mount(WechatCallbackView, {
+      global: {
+        stubs: {
+          AuthLayout: { template: '<div><slot /></div>' },
+          Icon: true,
+          RouterLink: { template: '<a><slot /></a>' },
+          transition: false,
+        },
+      },
+    })
+
+    await flushPromises()
+
+    await wrapper.get('[data-testid="existing-account-submit"]').trigger('click').catch(() => undefined)
+    await flushPromises()
+
+    expect(showErrorMock).toHaveBeenCalledWith('bind token failed')
+    expect(locationState.current.href).toBe('http://localhost/auth/wechat/callback')
+  })
+
   it('collects email, password, and verify code for pending oauth account creation and submits adoption decisions', async () => {
     getPublicSettingsMock.mockResolvedValue({
       invitation_code_enabled: true,