feat(ext): shared state host — decouple components from popup.ts

Introduce shared/state.ts as a service-locator so popup components
(item-detail, item-form, trash, devices, settings, etc.) work in both
the popup and vault tab bundles. Both entry points register themselves
as the host; components import from shared/state instead of popup.ts.
Vault.ts now delegates to the real popup components, removing ~300 lines
of placeholder renderers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

extension/src/popup/components/__tests__/attachments-disclosure.test.ts

@@ -1,13 +1,13 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 
-vi.mock('../../popup', async () => {
+vi.mock('../../../shared/state', async () => {
   const sendMessage = vi.fn();
   const escapeHtml = (s: string): string => s.replace(/[&<>"']/g, (c) => ({ '&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;', "'": '&#39;' }[c]!));
-  return { sendMessage, escapeHtml };
+  return { sendMessage, escapeHtml, popOutToTab: vi.fn(), isInTab: vi.fn(() => false), openVaultTab: vi.fn() };
 });
 
 import { renderAttachmentsDisclosure, wireAttachmentsDisclosure } from '../attachments-disclosure';
-import { sendMessage } from '../../popup';
+import { sendMessage } from '../../../shared/state';
 import type { AttachmentRef } from '../../../shared/types';
 
 const REF1: AttachmentRef = { id: 'a1', filename: 'doc.pdf', mime_type: 'application/pdf', size: 12345, created: 1700000000 };