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/e2e/reader-mode.spec.ts

@@ -201,13 +201,13 @@ test('reader-mode preference set on one page is honored when the reader opens',
     );
 });
 
-test('settings page hides the gap picker while in single-page mode', async ({ page }) => {
+test('preferences page hides the gap picker while in single-page mode', async ({ page }) => {
     // Visually verifies the conditional render. The radio-click semantics
     // are exercised in src/lib/preferences.svelte.test.ts; the visible
     // mode toggle in the reader top bar covers the cross-route propagation
     // path in the test above.
     await mockReaderApis(page);
-    await page.goto('/settings');
+    await page.goto('/profile/preferences');
 
     await expect(page.getByTestId('reader-mode-radio-single')).toBeAttached();
     await expect(page.getByTestId('reader-mode-radio-continuous')).toBeAttached();