feat: DM relay infrastructure and private zaps

[barrydeen]

Summary

• Apply default DM relays (kind 10050) for users with none configured and broadcast relay lists to indexers on startup
• Cache DM relay lists (kind 10050) in RelayListRepository alongside kind 10002 relay metadata
• Route private zap requests exclusively through DM relays so receipts stay off public relays
• Add private zap toggle in ZapDialog (mutually exclusive with anonymous), gated on both parties having DM relays
• Detect private zaps by matching zap request relay URLs against cached DM relay sets
• Add long-press zap inspector dialog showing receipt details, relay provenance, and zap request relay tags
• Subscribe to DM relays for kind 9735 receipts at startup and during feed engagement to ensure private zaps persist across sessions
• Replace Triple with ZapDetail data class for cleaner zap tracking throughout the codebase

Test plan

• Verify default DM relays are applied for new users with no kind 10050 set
• Send a private zap to a user with DM relays — confirm 9734 relay tag only contains DM relay URLs
• Verify private toggle is hidden when recipient has no DM relays
• Verify anonymous and private toggles are mutually exclusive
• Long-press a zap in notifications or reaction details to open inspector
• Verify private zap receipts persist after logout/login
• Verify private zap icon (mask) appears next to zaps in notifications and reaction details