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_docs_test.go

@@ -87,6 +87,8 @@ func TestAPIRoutesDocumented(t *testing.T) {
 			basePath = "/panel/api"
 		case "inbound.go":
 			basePath = "/panel/api/inbounds"
+		case "client.go":
+			basePath = "/panel/api/clients"
 		case "server.go":
 			basePath = "/panel/api/server"
 		case "node.go":
@@ -127,6 +129,7 @@ func TestAPIRoutesDocumented(t *testing.T) {
 		// Skip SPA page routes (these are UI pages, not API endpoints)
 		spaPages := map[string]bool{
 			"/": true, "/panel/": true, "/panel/inbounds": true,
+			"/panel/clients": true,
 			"/panel/nodes": true, "/panel/settings": true,
 			"/panel/xray": true, "/panel/api-docs": true,
 		}