feat(clients): add top-level Clients tab and CRUD API

Adds /panel/api/clients endpoints (list, get, add, update, del,
attach, detach) backed by ClientService methods that orchestrate
the per-inbound Add/Update/Del flows so a single client row is
created once and attached to many inbounds in one operation.

The frontend gains a dedicated Clients page (frontend/clients.html
+ src/pages/clients/) with an AntD table, multi-inbound attach
modal, and full CRUD. Axios interceptor learns to honour
Content-Type: application/json so the JSON endpoints work
alongside the legacy form-encoded ones.

The legacy per-inbound client modal stays untouched in this PR —
both flows now write to the same source of truth.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/AppSidebar.vue

@@ -4,6 +4,7 @@ import { useI18n } from 'vue-i18n';
 import {
   DashboardOutlined,
   UserOutlined,
+  TeamOutlined,
   SettingOutlined,
   ToolOutlined,
   ClusterOutlined,
@@ -30,6 +31,7 @@ const props = defineProps({
 const iconByName = {
   dashboard: DashboardOutlined,
   user: UserOutlined,
+  team: TeamOutlined,
   setting: SettingOutlined,
   tool: ToolOutlined,
   cluster: ClusterOutlined,
@@ -42,6 +44,7 @@ const prefix = props.basePath?.startsWith('/') ? props.basePath : `/${props.base
 const tabs = computed(() => [
   { key: `${prefix}panel/`, icon: 'dashboard', title: t('menu.dashboard') },
   { key: `${prefix}panel/inbounds`, icon: 'user', title: t('menu.inbounds') },
+  { key: `${prefix}panel/clients`, icon: 'team', title: t('menu.clients') },
   { key: `${prefix}panel/nodes`, icon: 'cluster', title: t('menu.nodes') },
   { key: `${prefix}panel/settings`, icon: 'setting', title: t('menu.settings') },
   { key: `${prefix}panel/xray`, icon: 'tool', title: t('menu.xray') },