Add OAuth2 dynamic client registration toggle to the deployment settings UI #27432

#27316 adds an admin toggle for OAuth2 Dynamic Client Registration (RFC 7591): GET/PUT /api/v2/oauth2-provider/settings with codersdk.OAuth2ProviderSettings{dynamic_client_registration_enabled}, default disabled. It is currently only reachable via the API or coder oauth2-provider dcr enable|disable.

Add the setting to the deployment settings UI:

• Surface the toggle on an appropriate deployment settings page (owner-only, matching the endpoint's ResourceDeploymentConfig authz).
• OAuth2ProviderSettings is already exported in site/src/api/typesGenerated.ts.
• Changes are audited server-side (oauth2_provider_settings resource type), so no extra UI audit work beyond the usual display.
• Note the default-disabled semantics: never-configured reads return dynamic_client_registration_enabled: false.

_{Coder Agents on behalf of @Emyrk.}