test(e2e): cover configurable destinations; stop assuming a nav item

The assertions are about the navigation, not about the setting: "the
config was saved" is the plumbing, and #69 and #72 both shipped green
under specs that measured exactly that.

Four existing specs reached a view by clicking its nav item, which since
this change is not guaranteed to exist -- Autotag is hidden by default
and Downloads is absent without a download client -- so they timed out
waiting for a locator that will never resolve. `navigateTo` dispatches
the app's own `navigate` event, which is what every nav item, card and
detail view dispatches, so it is the mechanism rather than a test-only
door. Click the item when the nav is the subject.

Closes #25

e2e/specs/settings-reach.spec.ts

@@ -1,4 +1,4 @@
-import { test, expect } from '../support/fixtures.js';
+import { test, expect, navigateTo } from '../support/fixtures.js';
 
 /**
  * Plan 007 phase 5: a11y.1 and a11y.2, frozen against the real app.
@@ -56,7 +56,9 @@ test.describe('Settings is reachable without a mouse', () => {
 
 test.describe("Downloads' tabs are tabs", () => {
   test('arrow keys move the selection and swap the panel', async ({ app }) => {
-    await app.getByTestId('nav-downloads').click();
+    // By event, not by nav item: with no download client configured
+    // there is no Downloads destination to click (#25).
+    await navigateTo(app, 'downloads');
 
     const view = app.locator('downloads-view');
     const requests = view.getByRole('tab', { name: 'Requests' });