feat: add lastMessageText and lastMessageChannel to contacts query

Enriches the contacts resolver to include the last message preview
and channel, so the sidebar can show thread previews without loading
all communications. No frontend changes yet — fields returned but unused.

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

frontend/graphql/operations/contacts.graphql

@@ -5,6 +5,8 @@ query ContactsQuery {
     avatar
     channels
     lastContactAt
+    lastMessageText
+    lastMessageChannel
     description
   }
 }

frontend/graphql/schema.graphql

@@ -171,6 +171,8 @@ type Contact {
   avatar: String!
   channels: [String!]!
   lastContactAt: String!
+  lastMessageText: String!
+  lastMessageChannel: String!
   description: String!
 }