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/wailsjs/go/explore/Service.d.ts

@@ -21,6 +21,8 @@ export function GetArtistMBID(arg1:string):Promise<string>;
 
 export function GetArtistPlayCount(arg1:string):Promise<number>;
 
+export function GetLibrarySimilarArtists(arg1:string):Promise<Array<explore.LBSimilarArtist>>;
+
 export function GetThumbnail(arg1:string,arg2:string,arg3:string):Promise<string>;
 
 export function GetThumbnails(arg1:Array<explore.ThumbnailRequest>):Promise<Record<string, string>>;

frontend/wailsjs/go/explore/Service.js

@@ -38,6 +38,10 @@ export function GetArtistPlayCount(arg1) {
   return window['go']['explore']['Service']['GetArtistPlayCount'](arg1);
 }
 
+export function GetLibrarySimilarArtists(arg1) {
+  return window['go']['explore']['Service']['GetLibrarySimilarArtists'](arg1);
+}
+
 export function GetThumbnail(arg1, arg2, arg3) {
   return window['go']['explore']['Service']['GetThumbnail'](arg1, arg2, arg3);
 }