fix(local-llm): restore custom GGUF setup without restart (#417)

* fix(local-llm): restore custom gguf downloads and startup registration

* fix(local-llm): address PR review feedback

* test(sandbox): fix off mode coverage

* fix(web): unhide local model progress modal

* fix(local-llm): address review feedback

* fix(local-llm): preserve colliding provider entries

* fix(gateway): retain custom local models after discovery

* fix(local-llm): harden custom model validation and cleanup

* fix(gateway): honor local disable flag during restore

* fix(gateway): preserve local model restore state

* test(web-ui): stabilize voice fallback websocket e2e

Author: penso