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>

plugins/registry.json

@@ -0,0 +1,7 @@
+[
+  {
+    "repo": "HakanSeven12/opencad-example-plugin",
+    "name": "Example Plugin",
+    "description": "Reference dynamically-loaded add-on (ribbon tab + EX_HELLO command)."
+  }
+]