Add a customer-facing toggle to enable Fast mode models

Follow-up to github/copilot-api#37129, item 4.

Customers need a self-service way to enable Fast mode from the model picker. Today, enablement requires support or internal intervention, which prevents customers from discovering and using the feature directly.

Add a **Fast mode** section to the model picker with an **Enable fast mode** toggle. The UI should:

- Reflect whether Fast mode is available and currently enabled for the signed-in account.
- Let an eligible customer enable it directly.
- Explain that Fast mode requires usage credits before the customer confirms enablement.
- Show a clear disabled state and reason when the account is not eligible.
- Refresh model availability after enablement without requiring a restart or sign-out.
- Keep the state synchronized across windows and devices as supported by the service.
- Support keyboard navigation and provide an accessible label and state announcement.

This issue should also identify and wire up the required API/entitlement contract with the Copilot service team.

**Acceptance criteria**

1. An eligible customer can enable Fast mode from the model picker and use it immediately.
2. The toggle accurately reflects the account-level state after reload and in other windows.
3. Ineligible customers see why the control is unavailable rather than a non-functional toggle.
4. Usage-credit implications are communicated before enablement.
5. Enablement failures surface an actionable error and do not leave the UI in an incorrect state.

**Proposed UI**