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/shared/commands.ts

@@ -89,6 +89,12 @@ export interface CreateChatSpaceInput {
   visibility: "workspace" | "restricted";
 }
 
+export interface MarkChatSpaceReadInput {
+  orgId: string;
+  employeeId: string;
+  spaceId: string;
+}
+
 export interface SaveProviderCredentialInput {
   orgId: string;
   providerId: string;