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/schema.graphql

@@ -33,6 +33,7 @@ type Mutation {
   updateCommunicationTranscript(id: ID!, transcript: [String!]!): MutationWithIdResult!
   updateFeedDecision(id: ID!, decision: String!, decisionNote: String): MutationWithIdResult!
   setContactInboxHidden(inboxId: ID!, hidden: Boolean!): MutationResult!
+  markThreadRead(contactId: ID!): MutationResult!
 }
 
 type MutationResult {
@@ -173,6 +174,7 @@ type Contact {
   lastContactAt: String!
   lastMessageText: String!
   lastMessageChannel: String!
+  hasUnread: Boolean!
   description: String!
 }