feat: default-sort the website table by downloads per month

Entries with a download count now sort first (descending), with
stars, then Built-in, then name as fallback tiers for entries that
lack a count. main.js mirrors this in its default activeSort, clean
URL check, and third-click reset target. Sorting by stars remains one
header click away.

Co-Authored-By: Claude <noreply@anthropic.com>

website/tests/test_build.py

@@ -463,6 +463,8 @@ class TestBuild:
         assert "1,234,567" in html
         # Built-in entries never show PyPI counts: the asyncio row is the backport package
         assert "26,305,454" not in html
+        # Default sort: entries with download counts come first
+        assert html.index("My-Lib") < html.index("no-pypi")
 
     def test_build_fails_when_group_and_category_slug_collide(self, tmp_path):
         readme = textwrap.dedent("""\