Remove duplicates in the web interface

The settings page has one Save button helper. The story list sorts by
the sort key that the server sends, so the rule for a leading article
exists once. The story page takes its list of download modes from the
mode hints.

tests/test_store.py

@@ -55,6 +55,8 @@ def test_list_stories_sorts_titles_without_leading_articles(tmp_path):
         store.create_story(f"https://example/s/{n}", "example", title, "A", [])
     titles = [s["title"] for s in store.list_stories()]
     assert titles == ["Anthem", "A Bee", "Cat", "an Owl", "Theory", "The Zebra"]
+    # The web interface sorts by the same key.
+    assert [s["sort_title"] for s in store.list_stories()] == ["anthem", "bee", "cat", "owl", "theory", "zebra"]
 
 
 def test_cover_replaces_previous_cover(tmp_path):