fix(v2): Fix DM contact names showing as pubkey prefix

- DM handler: don't overwrite contact name with prefix when name unknown
- Migration: upsert contact with sender name from v1 PRIV entries
- Fixes conversations showing "4e45565e" instead of "demo mc-webui"

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

app/device_manager.py

@@ -312,19 +312,20 @@ class DeviceManager:
             sender_key = data.get('public_key', data.get('pubkey_prefix', ''))
 
             # Look up sender from contacts — resolve prefix to full public key
-            sender_name = 'Unknown'
+            sender_name = ''
             if sender_key and self.mc:
                 contact = self.mc.get_contact_by_key_prefix(sender_key)
                 if contact:
-                    sender_name = contact.get('name', sender_key[:8])
+                    sender_name = contact.get('name', '')
                     # Use the full public key from contacts (not the short prefix)
                     full_key = contact.get('public_key', '')
                     if full_key:
                         sender_key = full_key
             if sender_key:
+                # Only upsert with name if we have a real name (not just a prefix)
                 self.db.upsert_contact(
                     public_key=sender_key,
-                    name=sender_name,
+                    name=sender_name,  # empty string won't overwrite existing name
                     source='message',
                 )
 
@@ -340,13 +341,13 @@ class DeviceManager:
                 raw_json=json.dumps(data, default=str),
             )
 
-            logger.info(f"DM #{dm_id} from {sender_name}")
+            logger.info(f"DM #{dm_id} from {sender_name or sender_key[:12]}")
 
             if self.socketio:
                 self.socketio.emit('new_message', {
                     'type': 'dm',
                     'contact_pubkey': sender_key,
-                    'sender': sender_name,
+                    'sender': sender_name or sender_key[:12],
                     'content': content,
                     'timestamp': ts,
                     'id': dm_id,

app/migrate_v1.py

@@ -254,12 +254,21 @@ def _migrate_dm_incoming(db, entry: dict):
         return
 
     pubkey_prefix = entry.get('pubkey_prefix', '')
+    sender_name = entry.get('name', '')
 
-    # Use None if pubkey not in contacts table (FK constraint)
+    # Resolve prefix to full key if contact exists
     contact_key = pubkey_prefix if pubkey_prefix else None
     if contact_key:
         contact_key = _resolve_pubkey(db, contact_key)
 
+    # Create/update contact with sender name from v1 data
+    if contact_key and sender_name:
+        db.upsert_contact(
+            public_key=contact_key,
+            name=sender_name,
+            source='message',
+        )
+
     db.insert_direct_message(
         contact_pubkey=contact_key,
         direction='in',