feat(plugin): enable/disable plugins from the Plugin Manager

The manager was a read-only inventory. Each plugin now has an
Enable/Disable toggle: a disabled plugin keeps its manifest listed but
drops its ribbon tab and command dispatch. The disabled-id set persists
in settings.txt (disabled_plugins=) and the ribbon rebuilds on toggle.

Part of the #100 extensibility epic.

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

src/app/view.rs

@@ -1120,7 +1120,10 @@ impl OpenCADStudio {
                 sized(crate::ui::shortcuts::view_window(&self.shortcut_overrides), 720, 520)
             }
             super::ModalKind::PluginManager => sized(
-                crate::ui::plugin_manager::view_window(&crate::plugin::installed_manifests()),
+                crate::ui::plugin_manager::view_window(
+                    &crate::plugin::installed_manifests(),
+                    &self.disabled_plugins,
+                ),
                 520,
                 460,
             ),