Add per-session reasoning effort support in WebUI

Summary

Please add per-session reasoning effort support to the WebUI, so it's possible to set reasoning for one conversation without changing the global/default setting for other sessions.

Problem

Right now, reasoning effort appears to be treated as a shared/global setting in the WebUI flow.

Desired behavior

• Changing reasoning effort in one session should only affect that session.
• New sessions should continue using the configured default unless explicitly changed.

Notes

This should mirror the session-scoped behavior available in Hermes Agent, where reasoning effort can be set per session while still falling back to a global default.