feat: add schedule selector to mobile Added Pane (#1177)

[ronaldwen07]

Summary

Adds a schedule selector to the mobile Added Pane, allowing mobile users to switch between schedules. The selector only appears on mobile viewports and reuses the existing SelectSchedulePopover component.

Test Plan

1. Open AntAlmanac on localhost:3000
2. Resize browser window to mobile width (<600px) or use device emulation
3. Navigate to the "Added" courses tab
4. Verify schedule selector button appears next to schedule name (e.g., "Schedule 1 (0 Units)")
5. Click the selector and verify you can switch between schedules
6. Resize to desktop width and verify selector does NOT appear (desktop uses toolbar selector)

Issues

Closes #1177