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-tauri/src/commands/settings_export_commands.rs

@@ -441,6 +441,13 @@ pub async fn apply_settings_import(
         ) = imported_marketplace;
         state.settings_store.update(s)?
     };
+    // Caches of marketplaces the import dropped are dead weight now, and
+    // the pins must match the imported installs.
+    use crate::commands::marketplace_commands as mc;
+    for id in mc::dropped_marketplace_ids(&current, &saved) {
+        mc::remove_cache(&state, &id).await;
+    }
+    mc::refresh_pins(&state).await;
 
     // `reconcile_gateway` (inside `update_settings`) only reacts to a changed
     // *shape* — port, provider, base URL, models — because that's what's