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

frontend/src/pages/PackagePage.css

@@ -849,13 +849,20 @@ tr:hover .copy-btn {
   position: relative;
 }
 
-.action-menu-dropdown {
-  position: absolute;
-  top: 100%;
+/* Action menu backdrop for click-outside */
+.action-menu-backdrop {
+  position: fixed;
+  top: 0;
+  left: 0;
   right: 0;
-  z-index: 100;
+  bottom: 0;
+  z-index: 999;
+}
+
+.action-menu-dropdown {
+  position: fixed;
+  z-index: 1000;
   min-width: 180px;
-  margin-top: 4px;
   padding: 4px 0;
   background: var(--bg-secondary);
   border: 1px solid var(--border-primary);