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/fields.ts

@@ -5,7 +5,7 @@
 /// After mounting, call `wireFieldHandlers(scope)` once to bind reveal +
 /// copy click handlers on any rendered rows.
 
-import { escapeHtml } from '../popup';
+import { escapeHtml } from '../../shared/state';
 import type { Item, Section, Field, FieldValue } from '../../shared/types';
 
 export interface RowOpts {