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/preload/index.ts

@@ -17,6 +17,7 @@ const api: DesktopApi = {
   decideApproval: (input) => invoke(IPC_CHANNELS.decideApproval, input),
   createChatSpace: (input) => invoke(IPC_CHANNELS.createChatSpace, input),
   sendMessage: (input) => invoke(IPC_CHANNELS.sendMessage, input),
+  markChatSpaceRead: (input) => invoke(IPC_CHANNELS.markChatSpaceRead, input),
   saveProviderCredential: (input) => invoke(IPC_CHANNELS.saveProviderCredential, input),
   createProviderResponse: (input) => invoke(IPC_CHANNELS.createProviderResponse, input),
   importSkillDirectory: (input) => invoke(IPC_CHANNELS.importSkillDirectory, input),