Add NIP-17 kind 10050 (DM relay list) support for reliable cross-client DM delivery #43

Problem

Vector does not implement NIP-17 kind 10050 (DM relay list) support. This means:

• No publishing: Vector never publishes a kind 10050 event to advertise where it receives DMs
• No fetching: Vector never looks up a recipient's kind 10050 to discover their preferred DM inbox relays
• No targeted routing: DM gift wraps are broadcast to all pool relays via client.gift_wrap() instead of being sent to the recipient's preferred relays

Current behavior

• message/sending.rs:852 — DM sends use client.gift_wrap() which broadcasts to all connected pool relays
• Cross-client DMs (e.g. with 0xchat) only work if there happens to be relay overlap (e.g. relay.damus.io)
• If a recipient uses DM-specific inbox relays (common with NIP-17 compliant clients), messages may never arrive

Evidence that targeted delivery is already possible

gift_wrap_to() is already used in Vector for:

• Typing indicators → commands/realtime.rs:50
• WebXDC peer ads → commands/realtime.rs:137

The same pattern should be applied to DM message sends.

Proposed Solution

1. Publish kind 10050 on login/relay config change — advertise Vector's connected relays as DM inbox relays
2. Fetch recipient's kind 10050 before sending a DM — look up their preferred DM relays
3. Use client.gift_wrap_to(recipient_dm_relays, ...) instead of client.gift_wrap(...) for DM sends
4. Fallback: if no kind 10050 is found for the recipient, fall back to current broadcast behavior

Impact

Without this, DM interoperability with other NIP-17 compliant clients (0xchat, etc.) is unreliable and depends on coincidental relay overlap rather than proper protocol-level relay discovery.

References

• NIP-17: Private Direct Messages — see "Relays" section on kind 10050
• Affects both Vector (Tauri app) and Vector-SDK