feat: MBIDs in library models + local-first search + explore cache

Backend:
- Added mbid column to sqlc schemas for artists and release_groups
- Regenerated sqlc queries to SELECT mbid in artist/album queries
- Added MBID field to library.Artist and library.Album Go structs
- All GetAllArtists/GetAllAlbums variants now populate MBID

Frontend:
- Updated Wails models.ts with MBID fields on Artist and Album
- Added cachedArtists/cachedAlbums getters to LibraryStore
- searchLibraryCache now includes MBIDs and local cover art URLs
  so library results can navigate to explore detail pages
- Added mergeWithLibrary() — when full MB results arrive, library
  entries are enriched with local images and 'In Library' flags
  rather than being replaced by MB-only versions
- Created ExploreCache store for cross-page data sharing: search
  results populate the cache, detail pages can read from it to
  avoid redundant API calls for already-fetched data

frontend/wailsjs/go/models.ts

@@ -242,6 +242,7 @@ export namespace library {
 	    ID: number;
 	    Name: string;
 	    ArtistName: string;
+	    MBID: string;
 	    CoverArtPath: string;
 	    CoverArtSmall: string;
 	    CoverArtMedium: string;
@@ -257,6 +258,7 @@ export namespace library {
 	        this.ID = source["ID"];
 	        this.Name = source["Name"];
 	        this.ArtistName = source["ArtistName"];
+	        this.MBID = source["MBID"];
 	        this.CoverArtPath = source["CoverArtPath"];
 	        this.CoverArtSmall = source["CoverArtSmall"];
 	        this.CoverArtMedium = source["CoverArtMedium"];
@@ -267,6 +269,7 @@ export namespace library {
 	export class Artist {
 	    ID: number;
 	    Name: string;
+	    MBID: string;
 	    ImageSmall: string;
 	    ImageMedium: string;
 	    ImageLarge: string;
@@ -279,6 +282,7 @@ export namespace library {
 	        if ('string' === typeof source) source = JSON.parse(source);
 	        this.ID = source["ID"];
 	        this.Name = source["Name"];
+	        this.MBID = source["MBID"];
 	        this.ImageSmall = source["ImageSmall"];
 	        this.ImageMedium = source["ImageMedium"];
 	        this.ImageLarge = source["ImageLarge"];