Marketplace: warn on imported global plugins; tidy import follow-ups

- The import preview counts global plugin installs and warns on them:
  a plugin can bring hooks and MCP servers into every container and an
  imported install skips the confirm step, like a hook.
- Item keys, hosts and branches in errors are quoted with {:?} and
  capped, since they can come from an import file.
- After an import, caches and snapshots of marketplaces the import
  dropped are removed (under the repo lock) and pins are refreshed for
  the imported installs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src/components/settings/ImportSettingsModal.test.tsx

@@ -34,6 +34,7 @@ const samplePreview: SettingsImportPreview = {
   custom_image_name: null,
   marketplace_count: 0,
   global_hook_install_count: 0,
+  global_plugin_install_count: 0,
   marketplace_account_token_count: 0,
 };