Fix thread-unsafe state writes; use conversation-agent dropdown

- entity.py: decorate the dispatcher target _handle_update with @callback so it
  runs on the event loop. Without it, async_dispatcher_send offloaded the plain
  sync callback to an executor thread, and async_write_ha_state raised
  "calls async_write_ha_state from a thread other than the event loop".
- config_flow.py: replace the free-text Assist agent field with
  ConversationAgentSelector (a proper agent dropdown), and use LanguageSelector
  for the language field.
- README: clarify that the built-in Assist agent only handles device commands;
  an LLM conversation agent is required for free-form chat.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

custom_components/reticulum/manifest.json

@@ -12,5 +12,5 @@
   "loggers": ["RNS", "LXMF"],
   "requirements": ["rns>=0.9.0", "lxmf>=0.6.0"],
   "single_config_entry": true,
-  "version": "1.0.3"
+  "version": "1.0.4"
 }