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/desktop-api.ts

@@ -8,6 +8,7 @@ import type {
   CreateApprovalInput,
   CreateChatSpaceInput,
   CreateMigrationReportInput,
+  MarkChatSpaceReadInput,
   CreateOrganizationInput,
   CreateProviderResponseInput,
   CreateScheduledPostInput,
@@ -40,6 +41,7 @@ export interface DesktopApi {
   decideApproval(input: DecideApprovalInput): Promise<DesktopResult<AppSnapshot>>;
   createChatSpace(input: CreateChatSpaceInput): Promise<DesktopResult<AppSnapshot>>;
   sendMessage(input: SendMessageInput): Promise<DesktopResult<AppSnapshot>>;
+  markChatSpaceRead(input: MarkChatSpaceReadInput): Promise<DesktopResult<AppSnapshot>>;
   saveProviderCredential(input: SaveProviderCredentialInput): Promise<DesktopResult<AppSnapshot>>;
   createProviderResponse(input: CreateProviderResponseInput): Promise<DesktopResult<AppSnapshot>>;
   importSkillDirectory(input: ImportSkillDirectoryInput): Promise<DesktopResult<AppSnapshot>>;