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/translation/en-US.json

@@ -94,6 +94,7 @@
     "ultraDark": "Ultra Dark",
     "dashboard": "Overview",
     "inbounds": "Inbounds",
+    "clients": "Clients",
     "nodes": "Nodes",
     "settings": "Panel Settings",
     "xray": "Xray Configs",
@@ -397,6 +398,19 @@
       "renew": "Auto Renew",
       "renewDesc": "Auto-renewal after expiration. (0 = disable)(unit: day)"
     },
+    "clients": {
+      "title": "Clients",
+      "addTitle": "Add Client",
+      "editTitle": "Edit Client",
+      "attachedInbounds": "Attached inbounds",
+      "selectInbound": "Select one or more inbounds",
+      "empty": "No clients yet — add one to get started.",
+      "deleteConfirmTitle": "Delete client {email}?",
+      "deleteConfirmContent": "This removes the client from every attached inbound and drops its traffic record. This cannot be undone.",
+      "toasts": {
+        "deleted": "Client deleted"
+      }
+    },
     "nodes": {
       "title": "Nodes",
       "addNode": "Add Node",