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 <[email protected]>

src/plugin/mod.rs

@@ -10,4 +10,4 @@ pub mod registry;
 
 pub use registry::all_ribbon_modules;
 pub(crate) use host::BuiltinPlugin;
-pub(crate) use registry::try_dispatch;
+pub(crate) use registry::{installed_manifests, try_dispatch};