feat: /profile dashboard with tabbed preferences, account, bookmarks, collections (0.18.0)

Tabbed user dashboard at `/profile` that absorbs `/settings` and
surfaces bookmarks + collections in one place.

- New `/profile` shell with tabs: Overview (counts), Preferences
  (theme + reader prefs, ported from /settings; works for guests
  via localStorage), Account (password change; auth-gated),
  Bookmarks, Collections. Guest tab list is filtered to what they
  can actually use.
- `/settings` is a 308 redirect to `/profile/preferences` so old
  bookmarks land cleanly. The "Settings" link in the top nav is
  replaced by a Profile link between Upload and Bookmarks; Bookmarks
  + Collections stay as shortcuts per the user spec.
- Extracts `lib/components/BookmarkList.svelte` and
  `lib/components/CollectionsGrid.svelte` so the top-level
  /bookmarks + /collections routes and the new profile tabs render
  the same UI without duplication. Both layers use a three-state
  load (authenticated / guest / error) to handle network hiccups
  inline.
- Deep links preserved via `?next=` on every sign-in CTA.

88 frontend unit tests + svelte-check clean; 12 of 12 e2e tests in
profile.spec.ts and reader-mode.spec.ts pass (8 other e2e failures
predate this branch and stay flagged for cleanup).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -6,9 +6,9 @@
     import { session } from '$lib/session.svelte';
     import { theme } from '$lib/theme.svelte';
     import Upload from '@lucide/svelte/icons/upload';
+    import UserCircle from '@lucide/svelte/icons/user-circle';
     import Bookmark from '@lucide/svelte/icons/bookmark';
     import FolderOpen from '@lucide/svelte/icons/folder-open';
-    import Settings from '@lucide/svelte/icons/settings';
     import LogOut from '@lucide/svelte/icons/log-out';
     import '$lib/styles/tokens.css';
 
@@ -53,6 +53,10 @@
             <Upload size={18} aria-hidden="true" />
             <span>Upload</span>
         </a>
+        <a class="nav-link" href="/profile" data-testid="nav-profile">
+            <UserCircle size={18} aria-hidden="true" />
+            <span>Profile</span>
+        </a>
         <a class="nav-link" href="/bookmarks">
             <Bookmark size={18} aria-hidden="true" />
             <span>Bookmarks</span>
@@ -67,10 +71,6 @@
             <span data-testid="session-loading" aria-busy="true">…</span>
         {:else if session.user}
             <span class="username" data-testid="session-user">{session.user.username}</span>
-            <a class="nav-link" href="/settings" data-testid="nav-settings">
-                <Settings size={18} aria-hidden="true" />
-                <span>Settings</span>
-            </a>
             <button
                 class="icon-btn"
                 type="button"