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/api.go

@@ -65,6 +65,9 @@ func (a *APIController) initRouter(g *gin.RouterGroup, customGeo *service.Custom
 	inbounds := api.Group("/inbounds")
 	a.inboundController = NewInboundController(inbounds)
 
+	clients := api.Group("/clients")
+	NewClientController(clients)
+
 	// Server API
 	server := api.Group("/server")
 	a.serverController = NewServerController(server)