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__/field-history.test.ts

@@ -2,7 +2,7 @@ import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { renderFieldHistory, teardown } from '../field-history';
 
 // Mock popup module
-vi.mock('../../popup', () => ({
+vi.mock('../../../shared/state', () => ({
   getState: vi.fn(() => ({
     historyItemId: 'item123',
     selectedItem: { id: 'item123', title: 'Test Item', modified: 1000 },
@@ -11,9 +11,12 @@ vi.mock('../../popup', () => ({
   sendMessage: vi.fn(),
   navigate: vi.fn(),
   escapeHtml: (s: string) => s,
+  popOutToTab: vi.fn(),
+  isInTab: vi.fn(() => false),
+  openVaultTab: vi.fn(),
 }));
 
-import { sendMessage, navigate } from '../../popup';
+import { sendMessage, navigate } from '../../../shared/state';
 
 describe('field-history view', () => {
   let app: HTMLElement;