fix: suppress all external API calls in library-only mode

Audit found three leaks:

1. explore-view: loadThumbnails() and loadArtistImages() fired on
   library search results. These call GetThumbnails (Wails RPC to
   Cover Art Archive proxy) and GetArtistImageURL (MB/Wikidata).
   Now skipped in library-only mode — library results already have
   local cover art and artist images from the library store.

2. explore-album-details: always called LookupReleaseGroup (MB) and
   BrowseReleases (MB) regardless of mode. Now skips both in
   library-only mode — shows only cache-hydrated header with no
   version selector or track listing.

3. explore-artist-details was already correct — the library-only
   branch skips all external calls.

frontend/src/components/explore-view/explore-view.ts

@@ -597,8 +597,12 @@ export class ExploreView extends LitElement {
                 localResults.artists || [],
                 localResults.releaseGroups || [],
             );
-            this.loadThumbnails();
-            this.loadArtistImages();
+            // In library-only mode, local results already have cover art
+            // and artist images from the library store — no API calls needed.
+            if (!exploreSettings.libraryOnly) {
+                this.loadThumbnails();
+                this.loadArtistImages();
+            }
             const elapsed = (performance.now() - startTime).toFixed(0);
             console.log(
                 `[explore] library results: "${query}" in ${elapsed}ms — ` +