feat(profile): add a desktop Page-tags section

Page tags were a first-class section of the mobile Library tab but had no
home in the desktop /profile tabs, so desktop users couldn't browse their
tagged pages without going through /search. Add a /profile/page-tags route
that reuses PageTagsList (the same component the mobile Library renders) and
a "Page tags" tab between Collections and History.

The loader mirrors the library wrapper's page-tags fetch; 401 → sign-in
prompt, matching the other profile sub-routes.

e2e: the desktop Page-tags tab navigates to /profile/page-tags and renders
the list.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/e2e/profile.spec.ts

@@ -50,6 +50,33 @@ test('Profile link in nav for authed users; landing shows counts', async ({ page
     await expect(page.getByTestId('overview-collections')).toBeVisible();
 });
 
+test('Page tags tab reaches the page-tags list on desktop', async ({ page }) => {
+    // Page tags are a first-class Library section on mobile; desktop reaches
+    // them through this profile tab.
+    await stubAuthenticated(page);
+    await page.route('**/api/v1/me/page-tags?*', (route) =>
+        route.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ items: [], page: { limit: 100, offset: 0, total: 0 } })
+        })
+    );
+    // Registered after the general route so distinct URLs resolve here.
+    await page.route('**/api/v1/me/page-tags/distinct*', (route) =>
+        route.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ items: [] })
+        })
+    );
+
+    await page.goto('/profile');
+    await expect(page.getByTestId('tab-page-tags')).toBeVisible();
+    await page.getByTestId('tab-page-tags').click();
+    await expect(page).toHaveURL(/\/profile\/page-tags$/);
+    await expect(page.getByTestId('library-page-tags-empty')).toBeVisible();
+});
+
 test('Account tab reaches the password form', async ({ page }) => {
     await stubAuthenticated(page);
     await page.goto('/profile');