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/ipc.ts

@@ -9,6 +9,7 @@ export const IPC_CHANNELS = {
   decideApproval: "agentcompany:decide-approval",
   createChatSpace: "agentcompany:create-chat-space",
   sendMessage: "agentcompany:send-message",
+  markChatSpaceRead: "agentcompany:mark-chat-space-read",
   saveProviderCredential: "agentcompany:save-provider-credential",
   createProviderResponse: "agentcompany:create-provider-response",
   importSkillDirectory: "agentcompany:import-skill-directory",