WebUI: Persist the on/off state of the MCP servers for new conversations (#20750)
* webui: add persistent storage for MCP server on/off state in new chats
* webui: simplify MCP enabled checks, remove dead server.enabled fallback
* chore: update webui build output
* chore: update webui build output
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Co-authored-by: Aleksander Grygier <aleksander.grygier@gmail.com>