feat(ext/settings): settings left-nav skeleton with section routing

Two-panel layout (148px nav sidebar + content area) with 7 nav items
(Autofill, Display, Security, Generator, Retention, Backup, Import),
stub section functions, and settings layout CSS classes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

extension/src/popup/components/__tests__/settings-nav.test.ts

@@ -0,0 +1,22 @@
+import { describe, it, expect, vi } from 'vitest';
+
+vi.stubGlobal('chrome', {
+  storage: {
+    local: {
+      get: vi.fn((_keys: unknown, cb: (r: Record<string, unknown>) => void) => cb({})),
+      set: vi.fn((_data: unknown, cb?: () => void) => cb?.()),
+    },
+  },
+});
+
+import * as settingsMod from '../settings';
+
+describe('settings module contract', () => {
+  it('exports renderSettings as a function', () => {
+    expect(typeof settingsMod.renderSettings).toBe('function');
+  });
+
+  it('exports teardownSettings as a function', () => {
+    expect(typeof settingsMod.teardownSettings).toBe('function');
+  });
+});