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/main/services/chat-service.ts

@@ -1,5 +1,5 @@
 import type { DatabaseSync } from "node:sqlite";
-import type { CreateChatSpaceInput, SendMessageInput } from "../../shared/commands";
+import type { CreateChatSpaceInput, MarkChatSpaceReadInput, SendMessageInput } from "../../shared/commands";
 import { createId } from "../../shared/ids";
 import { AuditService } from "./audit-service";
 
@@ -101,6 +101,28 @@ export class ChatService {
     return id;
   }
 
+  markSpaceRead(input: MarkChatSpaceReadInput): void {
+    const space = this.db.prepare("select id from chat_spaces where id = ? and org_id = ?")
+      .get(input.spaceId, input.orgId) as { id: string } | undefined;
+    if (!space) throw new Error("Chat space was not found.");
+    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.");
+    const latest = this.db.prepare(`
+      select id from chat_messages
+      where space_id = ? and org_id = ?
+      order by created_at desc
+      limit 1
+    `).get(input.spaceId, input.orgId) as { id: string } | undefined;
+    const now = new Date().toISOString();
+    this.db.prepare(`
+      insert into chat_read_state(org_id, employee_id, space_id, last_read_message_id, last_read_at)
+      values (?, ?, ?, ?, ?)
+      on conflict(employee_id, space_id)
+      do update set last_read_message_id = excluded.last_read_message_id, last_read_at = excluded.last_read_at
+    `).run(input.orgId, input.employeeId, input.spaceId, latest?.id ?? null, now);
+  }
+
   private recordMentions(messageId: string, body: string): void {
     const seen = new Set<string>();
     for (const match of body.matchAll(/(^|\s)@([A-Za-z][A-Za-z0-9_-]{1,63})/g)) {

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

@@ -64,6 +64,7 @@ export class SnapshotService {
       chatThreads: list(this.db, "chat_threads", "updated_at desc"),
       chatAttachments: list(this.db, "chat_attachments", "message_id asc"),
       chatPresence: list(this.db, "chat_presence", "last_seen_at desc"),
+      chatReadStates: list(this.db, "chat_read_state", "last_read_at desc"),
       files,
       scheduledPosts: list(this.db, "scheduled_posts", "scheduled_for asc"),
       providers,