Track per-employee chat read state and surface real unread counts

Adds chat_read_state migration (employee_id + space_id, last read
message + timestamp) wired through ChatService.markSpaceRead, IPC
markChatSpaceRead, and an effect in ChatPage that flushes the active
space whenever the latest message id changes.

Shell.computeBadges now derives the Messages badge from
(message.createdAt > read_state.last_read_at) minus messages the viewer
sent, so the sidebar shows true unread instead of total throughput.

AgentCompany-New/src/main/database/schema.ts

@@ -1,6 +1,6 @@
 import type { DatabaseSync } from "node:sqlite";
 
-const SCHEMA_VERSION = 4;
+const SCHEMA_VERSION = 5;
 
 export function installSchema(db: DatabaseSync): void {
   db.exec(`
@@ -484,5 +484,24 @@ export function installSchema(db: DatabaseSync): void {
       insert into schema_migrations(version, name, applied_at)
       values (4, 'files_pinned_and_scheduled_posts', datetime('now'));
     `);
+    currentVersion = 4;
+  }
+
+  if (currentVersion < 5) {
+    db.exec(`
+      create table if not exists chat_read_state (
+        org_id text not null references organizations(id) on delete cascade,
+        employee_id text not null references employees(id) on delete cascade,
+        space_id text not null references chat_spaces(id) on delete cascade,
+        last_read_message_id text references chat_messages(id) on delete set null,
+        last_read_at text not null,
+        primary key (employee_id, space_id)
+      );
+
+      create index if not exists idx_chat_read_state_org on chat_read_state(org_id, employee_id);
+
+      insert into schema_migrations(version, name, applied_at)
+      values (5, 'chat_read_state', datetime('now'));
+    `);
   }
 }