feat(routing): add client picker to user rules (#6271)

* feat(routing): add client picker to user rules

Replace the free-text user criterion with a searchable multi-select backed by existing panel clients. Preserve saved values that no longer exist so editing legacy rules remains lossless.

* feat(routing): polish user picker states

Align the routing user selector with the inbound-tag multi-select, including search, clear, loading, empty, and error states. Localize the new copy across every supported locale and cover legacy saved users with a regression test.

* fix(routing): keep custom user identifiers

Use tags mode with comma tokenization so the user picker suggests panel clients without rejecting HTTP, Mixed, or raw-template identifiers. Restore the comma hint and cover custom entries with a regression test.

internal/web/translation/fa-IR.json

@@ -1520,6 +1520,9 @@
         "attributes": "صفت‌ها",
         "value": "مقدار",
         "user": "کاربر",
+        "userPlaceholder": "انتخاب کاربران",
+        "userEmpty": "هیچ کاربری موجود نیست",
+        "userLoadError": "بارگیری کاربران ناموفق بود",
         "inboundTags": "تگ‌های ورودی",
         "outboundTag": "تگ خروجی",
         "balancerTag": "تگ بالانسر",