feat: show session ID in dashboard status display

Add session label (last 8 chars) next to each group status.
Export buildStatusContent and DashboardOptions for testing.

src/dashboard.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+
+import { buildStatusContent, type DashboardOptions } from './dashboard.js';
+
+function makeOptions(sessionId?: string): DashboardOptions {
+  const sessions: Record<string, string> = sessionId
+    ? { 'group-folder': sessionId }
+    : {};
+
+  return {
+    assistantName: 'Test',
+    channels: [],
+    getSessions: () => sessions,
+    queue: {
+      getStatuses: () => [
+        {
+          jid: 'dc:123',
+          status: 'inactive',
+          elapsedMs: null,
+          pendingMessages: false,
+          pendingTasks: 0,
+        },
+      ],
+    } as unknown as DashboardOptions['queue'],
+    registeredGroups: () => ({
+      'dc:123': {
+        name: 'clone-test',
+        folder: 'group-folder',
+        trigger: '@bot',
+        added_at: '2026-03-16T00:00:00.000Z',
+      },
+    }),
+    statusChannelId: 'status',
+    statusUpdateInterval: 60000,
+    usageUpdateInterval: 60000,
+  };
+}
+
+describe('buildStatusContent', () => {
+  it('shows cleared sessions as empty', () => {
+    const content = buildStatusContent(makeOptions());
+    expect(content).toContain('세션 없음');
+  });
+
+  it('shows a shortened session id when a session exists', () => {
+    const content = buildStatusContent(makeOptions('session-1234567890'));
+    expect(content).toContain('세션 34567890');
+  });
+});