Fix private zap toggle not detecting recipient DM relays

[barrydeen]

Summary

• Private zap toggle was disabled unless the recipient's DM relays (kind 10050) were already in the LRU cache, which only happened if you'd previously opened a DM conversation with them
• Now when the zap dialog opens, if the recipient's DM relays aren't cached, a live fetch is triggered from indexer relays (4s timeout) and the toggle updates reactively via LaunchedEffect
• Applied to all 5 ZapDialog call sites: feed, thread, article, notifications, and user profile screens

Test plan

• Open zap dialog on a note from someone you haven't DM'd — private toggle should enable after a brief moment if they have DM relays published
• Verify private zap toggle still works immediately when relays are already cached
• Verify private zap toggle stays disabled for users who genuinely have no DM relays
• Test from thread, article, notification, and profile screens