fix: relay confirmation before merge and welcome (MIP-02/MIP-03)

Rewrite add_member_device and remove_member_device to defer
merge_pending_commit until after relay confirmation, matching
MDK's documented ordering requirement.

Before: merge locally → fire-and-forget publish → send welcomes
After:  create commit → return immediately → background: relay
        publish with retries → merge → send welcomes → UI update

The Tauri command returns immediately (no UX blocking). A new
publish_event_with_retries helper provides 5-attempt exponential
backoff matching pika core's pattern.

This prevents permanent group state divergence when all relays
reject the commit, and ensures welcomes reference a commit that
is actually available on relays (MIP-02).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alltheseas

src-tauri/src/commands/mls.rs

@@ -653,8 +653,7 @@ pub async fn invite_member_to_group(
     .await
     .map_err(|e| format!("Task join error: {}", e))??;
 
-    // Sync participants array after adding members
-    sync_mls_group_participants(group_id).await?;
+    // Participant sync happens inside the background task after merge completes
 
     Ok(())
 }
@@ -681,8 +680,7 @@ pub async fn remove_mls_member_device(
     .await
     .map_err(|e| format!("Task join error: {}", e))??;
 
-    // Sync participants array after removing member
-    sync_mls_group_participants(group_id).await?;
+    // Participant sync happens inside the background task after merge completes
 
     Ok(())
 }