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/components/__tests__/field-history.test.ts

@@ -0,0 +1,69 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { renderFieldHistory, teardown } from '../field-history';
+
+// Mock popup module
+vi.mock('../../popup', () => ({
+  getState: vi.fn(() => ({
+    historyItemId: 'item123',
+    selectedItem: { id: 'item123', title: 'Test Item', modified: 1000 },
+  })),
+  setState: vi.fn(),
+  sendMessage: vi.fn(),
+  navigate: vi.fn(),
+  escapeHtml: (s: string) => s,
+}));
+
+import { sendMessage, navigate } from '../../popup';
+
+describe('field-history view', () => {
+  let app: HTMLElement;
+
+  beforeEach(() => {
+    app = document.createElement('div');
+    teardown();
+    vi.clearAllMocks();
+  });
+
+  it('renders empty state when no history', async () => {
+    (sendMessage as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ok: true,
+      data: { history: [] },
+    });
+
+    await renderFieldHistory(app);
+
+    expect(app.innerHTML).toContain('No history available');
+  });
+
+  it('renders history entries masked by default', async () => {
+    (sendMessage as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ok: true,
+      data: {
+        history: [{
+          field_id: 'f1',
+          field_name: 'password',
+          current_value: 'secret123',
+          entries: [{ value: 'oldpass', changed_at: 500 }],
+        }],
+      },
+    });
+
+    await renderFieldHistory(app);
+
+    expect(app.innerHTML).toContain('••••••••••••');
+    expect(app.innerHTML).not.toContain('secret123');
+    expect(app.innerHTML).toContain('current');
+  });
+
+  it('back button navigates to detail', async () => {
+    (sendMessage as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ok: true,
+      data: { history: [] },
+    });
+
+    await renderFieldHistory(app);
+    app.querySelector<HTMLButtonElement>('#back-btn')?.click();
+
+    expect(navigate).toHaveBeenCalledWith('detail');
+  });
+});