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.html

@@ -15,6 +15,10 @@
             <h1 class="title">YellowJacket</h1>
             <h3 class="subtitle">Music how it was meant to bee.</h3>
         </hgroup>
+        <button id="library-only-toggle" class="library-only-toggle" title="Library Only mode">
+            <wa-icon name="book"></wa-icon>
+            <span>Library Only</span>
+        </button>
         <library-filter></library-filter>
         <search-bar></search-bar>
     </header>