fix(frontend): accept unlimited (0) user concurrency in the edit dialog

The admin user edit modal rejected concurrency < 1, so a user whose
concurrency is already 0 could not be saved at all — the guard runs
before the request, blocking notes, password, role and RPM edits on that
user too.

Everywhere else already treats 0 as unlimited: the gateway skips slot
limiting when maxConcurrency <= 0 (ConcurrencyService.AcquireUserSlot),
the batch limits endpoint binds concurrency with min=0, and the bulk edit
modal only rejects negative values.

Reject negative and non-integer values instead, mirror the RPM field with
min/step and a "0 = unlimited" placeholder and hint, and rename the error
key to match its new meaning. Account concurrency is unchanged.

frontend/src/components/admin/user/UserEditModal.vue

@@ -43,7 +43,16 @@
       </div>
       <div>
         <label class="input-label">{{ t('admin.users.columns.concurrency') }}</label>
-        <input v-model.number="form.concurrency" type="number" class="input" />
+        <input
+          v-model.number="form.concurrency"
+          type="number"
+          min="0"
+          step="1"
+          class="input"
+          :placeholder="t('admin.users.form.concurrencyPlaceholder')"
+          data-test="concurrency-input"
+        />
+        <p class="input-hint">{{ t('admin.users.form.concurrencyHint') }}</p>
       </div>
       <div>
         <label class="input-label">{{ t('admin.users.form.rpmLimit') }}</label>
@@ -132,8 +141,9 @@ const handleUpdateUser = async () => {
     appStore.showError(t('admin.users.emailRequired'))
     return
   }
-  if (form.concurrency < 1) {
-    appStore.showError(t('admin.users.concurrencyMin'))
+  // 0 = 不限制，与网关 (AcquireUserSlot: maxConcurrency <= 0) 和批量改限额一致
+  if (!Number.isInteger(form.concurrency) || form.concurrency < 0) {
+    appStore.showError(t('admin.users.concurrencyNonNegative'))
     return
   }
   const userId = props.user.id

frontend/src/components/admin/user/__tests__/UserEditModal.spec.ts

@@ -0,0 +1,90 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { flushPromises, mount } from '@vue/test-utils'
+
+import UserEditModal from '../UserEditModal.vue'
+
+const { update, updateUserAttributeValues, showSuccess, showError } = vi.hoisted(() => ({
+  update: vi.fn(),
+  updateUserAttributeValues: vi.fn(),
+  showSuccess: vi.fn(),
+  showError: vi.fn()
+}))
+
+vi.mock('@/api/admin', () => ({
+  adminAPI: {
+    users: { update },
+    userAttributes: { updateUserAttributeValues }
+  }
+}))
+
+vi.mock('@/stores/app', () => ({
+  useAppStore: () => ({ showSuccess, showError })
+}))
+
+vi.mock('@/composables/useClipboard', () => ({
+  useClipboard: () => ({ copyToClipboard: vi.fn() })
+}))
+
+// useStepUp pulls in the API client, which needs the real i18n instance.
+vi.mock('vue-i18n', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('vue-i18n')>()),
+  useI18n: () => ({
+    t: (key: string, params?: Record<string, unknown>) =>
+      params ? `${key}:${JSON.stringify(params)}` : key
+  })
+}))
+
+const mountModal = (concurrency: number) => mount(UserEditModal, {
+  props: {
+    show: true,
+    user: { id: 7, email: 'user@example.test', username: 'user', notes: '', role: 'user', concurrency, rpm_limit: 0 } as never
+  },
+  global: {
+    stubs: {
+      BaseDialog: {
+        props: ['show', 'title'],
+        template: '<div v-if="show"><slot /><slot name="footer" /></div>'
+      },
+      Select: true,
+      Icon: true,
+      UserAttributeForm: true,
+      TotpStepUpDialog: true
+    }
+  }
+})
+
+describe('UserEditModal concurrency', () => {
+  beforeEach(() => {
+    update.mockReset()
+    updateUserAttributeValues.mockReset()
+    showSuccess.mockReset()
+    showError.mockReset()
+    update.mockResolvedValue({})
+  })
+
+  // Regression coverage for issue #5977: the gateway treats concurrency <= 0 as
+  // unlimited (AcquireUserSlot) and both the batch limits endpoint and the bulk
+  // edit modal accept 0, so this dialog must not be the only place that rejects
+  // it — doing so blocked every other edit on such a user.
+  it('saves an unlimited (0) concurrency instead of blocking the whole form', async () => {
+    const wrapper = mountModal(0)
+
+    await wrapper.get('form').trigger('submit')
+    await flushPromises()
+
+    expect(showError).not.toHaveBeenCalled()
+    expect(update).toHaveBeenCalledWith(7, expect.objectContaining({ concurrency: 0 }))
+    expect(wrapper.emitted('success')).toBeTruthy()
+  })
+
+  it('still rejects a negative concurrency', async () => {
+    const wrapper = mountModal(3)
+
+    await wrapper.get('[data-test="concurrency-input"]').setValue('-1')
+    await wrapper.get('form').trigger('submit')
+    await flushPromises()
+
+    expect(showError).toHaveBeenCalledWith('admin.users.concurrencyNonNegative')
+    expect(update).not.toHaveBeenCalled()
+  })
+})