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/store/explore-settings.ts

@@ -0,0 +1,41 @@
+/**
+ * ExploreSettingsStore — global settings for the explore feature.
+ * Persists to localStorage so the toggle state survives restarts.
+ */
+
+type Listener = () => void;
+
+class ExploreSettingsStore {
+    private _libraryOnly: boolean;
+    private listeners = new Set<Listener>();
+
+    constructor() {
+        this._libraryOnly = localStorage.getItem('explore:libraryOnly') === 'true';
+    }
+
+    get libraryOnly(): boolean {
+        return this._libraryOnly;
+    }
+
+    setLibraryOnly(value: boolean) {
+        if (this._libraryOnly === value) return;
+        this._libraryOnly = value;
+        localStorage.setItem('explore:libraryOnly', String(value));
+        this.notify();
+    }
+
+    toggle() {
+        this.setLibraryOnly(!this._libraryOnly);
+    }
+
+    subscribe(fn: Listener): () => void {
+        this.listeners.add(fn);
+        return () => this.listeners.delete(fn);
+    }
+
+    private notify() {
+        for (const fn of this.listeners) fn();
+    }
+}
+
+export const exploreSettings = new ExploreSettingsStore();