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.css

@@ -40,6 +40,37 @@ p {
     flex: 0 1 320px;
 }
 
+.library-only-toggle {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    padding: 6px 12px;
+    border: 1px solid var(--yj-border-subtle, rgba(255, 255, 255, 0.12));
+    border-radius: 6px;
+    background: transparent;
+    color: var(--yj-text-secondary, #b3b3b3);
+    font-size: 12px;
+    cursor: pointer;
+    transition: all 0.15s ease;
+    white-space: nowrap;
+    flex-shrink: 0;
+}
+
+.library-only-toggle:hover {
+    background: var(--yj-bg-overlay, rgba(255, 255, 255, 0.06));
+    color: var(--yj-text-primary, #fff);
+}
+
+.library-only-toggle.active {
+    background: var(--yj-accent, #ffd43b);
+    color: #000;
+    border-color: var(--yj-accent, #ffd43b);
+}
+
+.library-only-toggle wa-icon {
+    font-size: 14px;
+}
+
 ul {
     list-style-type: none;
 }