* 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.
