Improve PyPI proxy and Package page UX

PyPI proxy improvements:
- Set package format to "pypi" instead of "generic"
- Extract version from filename and create PackageVersion record
- Support .whl, .tar.gz, and .zip filename formats

Package page UX overhaul:
- Move upload to header button with modal
- Simplify table: combine Tag/Version, remove Type and Artifact ID columns
- Add row action menu (⋯) with: Copy ID, Ensure File, Create Tag, Dependencies
- Remove cluttered "Download by Artifact ID" and "Create/Update Tag" sections
- Add modals for upload and create tag actions
- Cleaner, more scannable table layout

frontend/src/pages/PackagePage.css

@@ -793,4 +793,133 @@ tr:hover .copy-btn {
   .ensure-file-modal {
     max-height: 90vh;
   }
+
+  .action-menu-dropdown {
+    right: 0;
+    left: auto;
+  }
+}
+
+/* Header upload button */
+.header-upload-btn {
+  margin-left: auto;
+}
+
+/* Tag/Version cell */
+.tag-version-cell {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+}
+
+.tag-version-cell .version-badge {
+  font-size: 0.75rem;
+  color: var(--text-muted);
+}
+
+/* Icon buttons */
+.btn-icon {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 32px;
+  height: 32px;
+  padding: 0;
+  background: transparent;
+  border: 1px solid transparent;
+  border-radius: var(--radius-sm);
+  color: var(--text-secondary);
+  cursor: pointer;
+  transition: all var(--transition-fast);
+}
+
+.btn-icon:hover {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+/* Action menu */
+.action-buttons {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+}
+
+.action-menu {
+  position: relative;
+}
+
+.action-menu-dropdown {
+  position: absolute;
+  top: 100%;
+  right: 0;
+  z-index: 100;
+  min-width: 180px;
+  margin-top: 4px;
+  padding: 4px 0;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border-primary);
+  border-radius: var(--radius-md);
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+}
+
+.action-menu-dropdown button {
+  display: block;
+  width: 100%;
+  padding: 8px 12px;
+  background: none;
+  border: none;
+  text-align: left;
+  font-size: 0.875rem;
+  color: var(--text-primary);
+  cursor: pointer;
+  transition: background var(--transition-fast);
+}
+
+.action-menu-dropdown button:hover {
+  background: var(--bg-hover);
+}
+
+/* Upload Modal */
+.upload-modal,
+.create-tag-modal {
+  background: var(--bg-secondary);
+  border-radius: var(--radius-lg);
+  width: 90%;
+  max-width: 500px;
+  max-height: 90vh;
+  overflow: hidden;
+}
+
+.modal-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 16px 20px;
+  border-bottom: 1px solid var(--border-primary);
+}
+
+.modal-header h3 {
+  margin: 0;
+  font-size: 1.125rem;
+  font-weight: 600;
+}
+
+.modal-body {
+  padding: 20px;
+}
+
+.modal-description {
+  margin-bottom: 16px;
+  color: var(--text-secondary);
+  font-size: 0.875rem;
+}
+
+.modal-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 12px;
+  margin-top: 20px;
+  padding-top: 16px;
+  border-top: 1px solid var(--border-primary);
 }