feat(ext/popup): field history view — masked values with reveal toggle

Shows current + historical values for tracked fields (password/concealed).
Click to reveal, copy button per entry (plaintext stored in a module-level
Map, never embedded in the DOM). Grouped by field name if multiple tracked
fields exist. Adds historyItemId to PopupState and 'field-history' to View.

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

extension/src/popup/popup.ts

@@ -11,6 +11,7 @@ import { renderItemDetail } from './components/item-detail';
 import { renderItemForm } from './components/item-form';
 import { renderSettings } from './components/settings';
 import { renderVaultSettings } from './components/settings-vault';
+import { renderFieldHistory } from './components/field-history';
 
 // --- Escape HTML to prevent XSS ---
 export function escapeHtml(str: string): string {
@@ -24,7 +25,7 @@ export function escapeHtml(str: string): string {
 
 // --- State ---
 
-export type View = 'locked' | 'list' | 'detail' | 'add' | 'edit' | 'settings' | 'settings-vault';
+export type View = 'locked' | 'list' | 'detail' | 'add' | 'edit' | 'settings' | 'settings-vault' | 'field-history';
 
 export interface PopupState {
   view: View;
@@ -45,6 +46,7 @@ export interface PopupState {
   newType: import('../shared/types').ItemType | null;
   vaultSettings: import('../shared/types').VaultSettings | null;
   generatorDefaults: import('../shared/types').GeneratorRequest | null;
+  historyItemId: import('../shared/types').ItemId | null;
 }
 
 let currentState: PopupState = {
@@ -62,6 +64,7 @@ let currentState: PopupState = {
   newType: null,
   vaultSettings: null,
   generatorDefaults: null,
+  historyItemId: null,
 };
 
 export function getState(): PopupState {
@@ -150,6 +153,9 @@ function render(): void {
     case 'settings-vault':
       renderVaultSettings(app);
       break;
+    case 'field-history':
+      renderFieldHistory(app);
+      break;
   }
 }