Summary

• Move isInstalling from InstalledPluginsView into PluginManager to prevent concurrent installs from any entry point (file picker, drag-and-drop, Finder double-click)
• Extract replaceExistingPlugin(bundleId:) helper shared by both installBundle and installFromZip to properly unload/unregister old plugins on reinstall
• Fix self-overwrite bug in installBundle when source URL is already in userPluginsDir
• Add missing replaceExistingPlugin call in installFromZip (was only in installBundle)
• Add CFBundleTypeExtensions to plugin document type for Finder association on fresh installs

Test plan

• Reinstalling an already-loaded plugin replaces it without duplicating the entry in Settings > Plugins
• Concurrent install attempts (e.g. drop while file picker install is running) are rejected
• Installing a plugin that already exists in userPluginsDir by path doesn't crash from self-deletion
• Zip-based plugin install still works correctly
• .tableplugin files show TablePro as the default handler in Finder