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/app/commands.rs

@@ -36,6 +36,7 @@ impl OpenCADStudio {
             && !matches!(
                 cmd,
                 "NEW" | "OPEN" | "EXIT" | "QUIT" | "REPORT" | "CHANGELOG" | "ABOUT"
+                    | "PLUGINS" | "PLUGINMANAGER"
             )
         {
             self.command_line
@@ -2675,6 +2676,10 @@ impl OpenCADStudio {
                 return Task::done(Message::AboutOpen);
             }
 
+            "PLUGINS" | "PLUGINMANAGER" => {
+                return Task::done(Message::PluginManagerOpen);
+            }
+
             "CHANGELOG" => {
                 let _ = open::that("https://github.com/HakanSeven12/OpenCADStudio/releases");
                 self.command_line.push_info("Opening release notes...");