feat(plugin): marketplace — install plugins from a curated registry

Add a Plugin Manager marketplace that installs external add-ons from a
GitHub repo's Releases. A curated registry (plugins/registry.json in this
repo, extended by PR) lists discoverable plugins; the host fetches it,
shows each entry with a release dropdown, and on install downloads the
asset matching the user's platform plus plugin.toml into the plugins
folder (API-version checked first). Users can also link arbitrary
owner/repo manually. Linked repos persist in settings.

Desktop only (ureq + serde_json); the install path reuses the phase-2
loader. Part of the #100 extensibility epic (phase 2).

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

src/app/view.rs

@@ -1126,6 +1126,14 @@ impl OpenCADStudio {
                     &self.disabled_plugins,
                     &self.external_plugins,
                     &self.loaded_plugin_ids,
+                    crate::ui::plugin_manager::MarketView {
+                        registry: &self.plugin_registry,
+                        input: &self.plugin_repo_input,
+                        repos: &self.plugin_repos,
+                        release_tags: &self.repo_release_tags,
+                        selected_tag: &self.repo_selected_tag,
+                        status: &self.marketplace_status,
+                    },
                 ),
                 520,
                 460,