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>

web/controller/xui.go

@@ -33,6 +33,7 @@ func (a *XUIController) initRouter(g *gin.RouterGroup) {
 
 	g.GET("/", a.index)
 	g.GET("/inbounds", a.inbounds)
+	g.GET("/clients", a.clients)
 	g.GET("/nodes", a.nodes)
 	g.GET("/settings", a.settings)
 	g.GET("/xray", a.xraySettings)
@@ -62,6 +63,10 @@ func (a *XUIController) inbounds(c *gin.Context) {
 	serveDistPage(c, "inbounds.html")
 }
 
+func (a *XUIController) clients(c *gin.Context) {
+	serveDistPage(c, "clients.html")
+}
+
 // nodes renders the multi-panel nodes management page.
 func (a *XUIController) nodes(c *gin.Context) {
 	serveDistPage(c, "nodes.html")