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/ja-JP.json

@@ -1520,6 +1520,9 @@
         "attributes": "属性",
         "value": "値",
         "user": "ユーザー",
+        "userPlaceholder": "ユーザーを選択",
+        "userEmpty": "利用可能なユーザーがいません",
+        "userLoadError": "ユーザーを読み込めませんでした",
         "inboundTags": "インバウンドタグ",
         "outboundTag": "アウトバウンドタグ",
         "balancerTag": "バランサータグ",