feat(plugin): discover external plugin packages from the plugins folder

Phase-2 groundwork: scan <config>/OpenCADStudio/plugins/<id>/plugin.toml,
parse the documented keys (no new TOML dep), check for a platform native
library beside it, and gate on the API version. The Plugin Manager gains
an "External (plugins folder)" section showing each package with a
status pill (Ready to load / API incompatible / No library). Discovery
only — actually loading the cdylib is the next step.

Part of the #100 extensibility epic (phase 2).

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

src/app/view.rs

@@ -1124,6 +1124,7 @@ impl OpenCADStudio {
                 crate::ui::plugin_manager::view_window(
                     &crate::plugin::installed_manifests(),
                     &self.disabled_plugins,
+                    &self.external_plugins,
                 ),
                 520,
                 460,