Adds an "Effort" slider to the chat model selector dropdown, matching the Figma design: label with an info tooltip, a discrete slider, and a value badge (e.g. "Xhigh"), pinned below the model list.

Behavior

• Rendered only when the selected model config has a max reasoning effort configured; slider steps span the provider's supported efforts up to the model's max.
• Initialized from the chat's last_reasoning_effort when valid, else the model's default; re-clamps when switching models.
• The selection is sent as reasoning_effort on chat creation, message send, and message edit (on edit only when the user changed the slider, so historical turns keep their original effort). The backend re-clamps regardless.
• Wired into both the chat page composer and the new-chat create form.

Part of the reasoning effort stack (base: #26975).

🤖 Generated by Coder Agents on behalf of @DanielleMaywood