feat: hashtag feed with kind 30015 interest sets

[barrydeen]

Summary

• Add NIP-51 interest set (kind 30015) support with InterestSet data class, parse/build helpers in Nip51.kt, and InterestRepository for multi-set storage with SharedPreferences persistence
• Wire through startup (fetch all sets), event routing, and full CRUD (follow/unfollow hashtag, create/rename/delete set) via ListCrudManager
• Add follow/unfollow bookmark button to HashtagFeedScreen with set picker when multiple sets exist
• Add "Hashtags" option to feed dropdown with grouped picker dialog showing sets with hashtag pills, inline rename, and delete confirmation

Test plan

• Launch app — verify kind 30015 events fetched at startup
• Open feed dropdown → tap "Hashtags" → verify empty state with create option
• Create a new interest set → verify kind 30015 event published
• Navigate to a hashtag feed → tap follow → verify set picker or direct add
• Verify hashtag appears as pill in grouped picker under correct set
• Rename a set → verify updated event published
• Delete a set → verify deletion event published
• Unfollow a hashtag → verify updated event published
• Kill and restart app → verify sets persist