feat: enhance feed follow list management

- Auto-assign user's default follow list to newly created feeds
- Prevent removal of user's own default follow list from feeds
- Redesign add dialog with two sections:
  - My Follow Lists: simple dropdown of user's own lists
  - Other Users' Follow Lists: user dropdown + follow list dropdown
- Add is_removable flag to feed follow list items with lock icon UI
- Add new API functions for the two-section add dialog

Closes #143