Defer LXMF client setup to gateway startup

[brothercorvo]

Summary

• lazily create the Emergency Management gateway LXMF client through a shared accessor that supports test overrides
• expose helper functions for resolving the server identity and reuse them when starting the link manager and reporting status
• attach/detach notification listeners during FastAPI startup and shutdown while resetting cached client state

Testing

• pytest tests/test_integration_webui_persistence.py

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Codex Task