feat: unread message tracking with blue dot indicator

Add ContactThreadRead model to track when users last viewed each contact thread.
Contacts with messages newer than the last read time show a blue dot in the sidebar.
Opening a thread automatically marks it as read via markThreadRead mutation.

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

frontend/graphql/operations/mark-thread-read.graphql

@@ -0,0 +1,5 @@
+mutation MarkThreadRead($contactId: ID!) {
+  markThreadRead(contactId: $contactId) {
+    ok
+  }
+}