feat: Library Only mode — toggle, search, artist page, similar artists

Backend:
- Migration 17: similar_artist_map table stores per-artist similar
  artist relationships (source_mbid → similar_mbid + name + score)
- Tier 4 index build now persists similar artists to this table
- GetLibrarySimilarArtists(mbid) queries similar artists filtered
  by JOIN with the artists table (library-only, no API calls)
- Added db field to explore.Service for direct queries

Frontend:
- ExploreSettingsStore with libraryOnly toggle, persisted to
  localStorage
- Top bar toggle button with active/inactive styling
- Explore search: skips full MB/LB pipeline when library-only,
  uses only searchLibraryCache (pure JS, instant)
- Artist detail page: in library-only mode, skips all API calls
  (no top tracks, no top releases, no LB play count, no MB
  artist lookup). Uses library store for discography, calls
  GetLibrarySimilarArtists for similar artists.
- Similar artists section: changed from horizontal scroll to
  wrapping flex layout with collapsible toggle (Show all N)
- Removed debug artist ranking log

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

@@ -11,6 +11,7 @@ import type {
 } from '@go/explore/Service';
 import { libraryStore } from '../../store/library-store';
 import { exploreCache } from '../../store/explore-cache';
+import { exploreSettings } from '../../store/explore-settings';
 import '@awesome.me/webawesome/dist/components/icon/icon.js';
 
 /* ── Constants ── */
@@ -592,7 +593,12 @@ export class ExploreView extends LitElement {
         }
 
         // Phase 2: full pipeline (MB + LB + reranking) via Wails RPC.
-        void this.executeFullSearch(version, query, startTime);
+        // Skip entirely in library-only mode — local results are final.
+        if (!exploreSettings.libraryOnly) {
+            void this.executeFullSearch(version, query, startTime);
+        } else {
+            this.loading = false;
+        }
     }
 
     /**