Add plugin manager UI

[luiox]

What's new

• Added Plugin Manager menu item and window for browsing, installing, and managing Recaf plugins
• New PluginMenu with actions: manage plugins, open dev guide, open template workspace
• PluginManagerPane with tabs for:
  • Browse — discover plugins from a repository index
  • Installed — view and manage installed plugins
  • Settings — configure plugin repository URLs and directories
• WindowManager integration with a new WIN_PLUGINS window identifier
• Translation entries for both �n_US and zh_CN
• Unit tests for the plugin manager pane
• Sample plugins (SampleAlphaPlugin, SampleBetaPlugin) for testing