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/index.ts

@@ -32,6 +32,7 @@ import '@store/theme-store';
 // Importing the keyboard shortcut service triggers initialization:
 // registers the document keydown listener for global shortcuts.
 import './src/services/keyboard-shortcut-service';
+import { exploreSettings } from '@store/explore-settings';
 import {
     hasTrackPayload,
     getDragPayload,
@@ -268,3 +269,24 @@ if (queueButton && queuePanel) {
 // or timing assumptions needed.
 void Player.EmitCurrentState();
 void Queue.EmitCurrentState();
+
+// ---------------------------------------------------------------------------
+// Library Only toggle
+// ---------------------------------------------------------------------------
+const libraryOnlyToggle = document.getElementById('library-only-toggle');
+
+if (libraryOnlyToggle) {
+    // Sync initial state.
+    if (exploreSettings.libraryOnly) {
+        libraryOnlyToggle.classList.add('active');
+    }
+
+    libraryOnlyToggle.addEventListener('click', () => {
+        exploreSettings.toggle();
+        libraryOnlyToggle.classList.toggle('active', exploreSettings.libraryOnly);
+    });
+
+    exploreSettings.subscribe(() => {
+        libraryOnlyToggle.classList.toggle('active', exploreSettings.libraryOnly);
+    });
+}