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/src/routes/profile/+layout.svelte

@@ -6,6 +6,7 @@
     import KeyRound from '@lucide/svelte/icons/key-round';
     import Bookmark from '@lucide/svelte/icons/bookmark';
     import FolderOpen from '@lucide/svelte/icons/folder-open';
+    import Tag from '@lucide/svelte/icons/tag';
     import History from '@lucide/svelte/icons/history';
 
     let { children } = $props();
@@ -27,6 +28,7 @@
         { href: '/profile/account', label: 'Account', icon: KeyRound, testid: 'tab-account', guestVisible: false },
         { href: '/profile/bookmarks', label: 'Bookmarks', icon: Bookmark, testid: 'tab-bookmarks', guestVisible: false },
         { href: '/profile/collections', label: 'Collections', icon: FolderOpen, testid: 'tab-collections', guestVisible: false },
+        { href: '/profile/page-tags', label: 'Page tags', icon: Tag, testid: 'tab-page-tags', guestVisible: false },
         { href: '/profile/history', label: 'History', icon: History, testid: 'tab-history', guestVisible: false }
     ];