Fix PyPI proxy UX and package stats calculation

- Fix artifact_count and total_size calculation to use Tags instead of
  Uploads, so PyPI cached packages show their stats correctly
- Fix PackagePage dropdown menu positioning (use fixed position with backdrop)
- Add system project detection for projects starting with "_"
- Show Version as primary column for system projects, hide Tag column
- Hide upload button for system projects (they're cache-only)
- Rename section header to "Versions" for system projects
- Fix test_projects_sort_by_name to exclude system projects from sort comparison

backend/tests/integration/test_projects_api.py

@@ -128,7 +128,9 @@ class TestProjectListingFilters:
         assert response.status_code == 200
 
         data = response.json()
-        names = [p["name"] for p in data["items"]]
+        # Filter out system projects (names starting with "_") as they may have
+        # collation-specific sort behavior and aren't part of the test data
+        names = [p["name"] for p in data["items"] if not p["name"].startswith("_")]
         assert names == sorted(names)