Turn the Messages rail into a real conversation list

Every channel and direct conversation is now its own row the way messengers
work: avatar or channel glyph, name, last-message preview, relative time,
and a true unread count from per-employee read state (the old badge showed
total message count). Rows sort by latest activity and the newest
conversation opens by default. Direct rows title themselves after the
counterpart resolved from memberships; agent-to-agent conversations the
board observes keep their two-name title, and opening one clears its unread
because read state is per-employee bookkeeping, not access control.
Conversation rows get their own .convo-item style instead of fighting the
sidebar's single-line nav-item rules.

AgentCompany-New/src/main/services/chat-service.ts

@@ -212,7 +212,9 @@ export class ChatService {
     const employee = this.db.prepare("select id from employees where id = ? and org_id = ?")
       .get(input.employeeId, input.orgId) as { id: string } | undefined;
     if (!employee) throw new Error("Employee was not found.");
-    this.requireMembership(input.spaceId, { type: "employee", id: input.employeeId });
+    // Read state is per-employee bookkeeping, not access control: the board
+    // observes every conversation in its org (agent↔agent directs included),
+    // so opening one clears its unread count even without membership.
     const latest = this.db.prepare(`
       select id from chat_messages
       where space_id = ? and org_id = ?