feat: NIP-51 encrypted private lists

[barrydeen]

Summary

• Add private list support for follow sets and bookmark sets using NIP-44 encrypted content
• Private list items are stored exclusively in the encrypted content field, never in public tags
• Privacy is set at creation time via a toggle in the create dialog; lock icon indicates private lists
• Own encrypted list events are decrypted on receipt in EventRouter, persisted with isPrivate flag

Test plan

• Create a private people list — verify the event sent has only ["d", "tag"] in tags with encrypted content
• Add/remove members to a private list — verify no p-tags leak into public tags
• Restart app — verify private lists load correctly from SharedPreferences and relay re-fetch
• Verify lock icon appears on private lists in Lists Hub and Add to List dialogs
• Create a public list — verify existing behavior unchanged (items in tags, empty content)
• Use a private list as feed — verify feed shows posts from decrypted members
• Add to private list from a user's profile — verify event remains encrypted