feat(plugin): add a Plugin Manager window listing installed add-ons

Read-only inventory of the add-ons compiled into the build — name, version,
id, API level, description, command prefixes — built from the extracted
`ocs_plugin_api` manifest. Opened with the `PLUGINS` / `PLUGINMANAGER` command
or the Start-page "Plugins" button.

Adds `installed_manifests()` to the plugin registry and wires the window the
same way as the existing About / Shortcuts windows.

Part of #100 (phase-1 plugin manager UI stub).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/mod.rs

@@ -17,6 +17,7 @@ pub mod open_progress;
 pub mod overlay;
 pub mod page_setup;
 pub mod plotstyle;
+pub mod plugin_manager;
 pub mod properties;
 pub mod ribbon;
 pub mod scale_popup;