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/item-form.ts

@@ -1,7 +1,7 @@
 /// Typed-item add/edit form dispatcher. Each type's renderForm lives in
 /// its own module under ./types/. Document stays "coming soon" until γ.
 
-import { navigate, getState, setState, escapeHtml, popOutToTab } from '../popup';
+import { navigate, getState, setState, escapeHtml, popOutToTab } from '../../shared/state';
 import type { Item, ItemType } from '../../shared/types';
 
 const TYPE_OPTIONS: Array<{ type: ItemType; icon: string; label: string }> = [