Add project-level Settings UI toggle for Duo auto mode (duo_auto_mode)

Track adding the project-level Auto mode (duo_auto_mode) toggle to Settings > GitLab Duo.

The group and admin UI shipped in Add duo_auto_mode UI toggle to Duo settings (!253317 - merged), which was scope-cut to group + admin and deliberately left out the project-level toggle, even though the project data model and API (project_settings.duo_auto_mode_enabled, cascading attr, GraphQL arg) already exist and mirror tool_approval_for_session.

This issue tracks closing that gap: the project Settings-page toggle (new project-setting-row in gitlab_duo_settings.vue + projects_helper wiring + cascading-lock parity).

• Implementing MR: Add duo_auto_mode toggle to project-level Duo s... (!256557 - merged)
• Epic: [Auto Mode] Iteration 0: Client-side enabled au... (&23199 - closed)