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-album-details/explore-album-details.ts

@@ -426,6 +426,14 @@ export class ExploreAlbumDetails extends LitElement {
             console.log(`[explore-album] hydrated from cache: "${cached.title}"`);
         }
 
+        // Library-only mode: no external API calls.
+        if (exploreSettings.libraryOnly) {
+            this.loadingInfo = false;
+            this.loadingReleases = false;
+            console.log(`[explore-album] loaded (library-only): "${this.albumName}"`);
+            return;
+        }
+
         // Phase 1: API calls for full data.
         const [infoResult, releasesResult] = await Promise.allSettled([
             this.fetchReleaseGroup(mbid),

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 — ` +