Marketplace: lock each cache on its own; a removed marketplace stays removed (PR review #5, #6)

The single global repo lock becomes one lock per marketplace, and
refresh_pins takes each marketplace's lock only while setting its pins, so
a slow fetch no longer queues installs and refreshes of other marketplaces.

refresh_marketplace now takes the lock first and reads the settings store
under it (a closure, not a copy captured earlier); a marketplace removed
meanwhile gets no cache and no snapshot. Removing a marketplace deletes its
snapshot and cache under the same lock (remove_marketplace_cache).

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

app/src-tauri/src/lib.rs

@@ -311,10 +311,14 @@ pub fn run() {
             // Failures are logged, not toasted — the Marketplace tab shows them.
             {
                 let settings = settings_store_setup.get();
+                let settings_store = settings_store_setup.clone();
                 let marketplace = marketplace_setup.clone();
                 tauri::async_runtime::spawn(async move {
                     for m in &settings.marketplaces {
-                        let snap = crate::marketplace::refresh_marketplace(&marketplace, &settings, &m.id).await;
+                        // Reads the store again under the lock: one removed
+                        // since startup is skipped (PR review #6).
+                        let current = || settings_store.get();
+                        let snap = crate::marketplace::refresh_marketplace(&marketplace, &current, &m.id).await;
                         if let Some(e) = snap.fetch_error {
                             log::warn!("Marketplace \"{}\" could not be refreshed at startup: {}", m.name, e);
                         }