perf: batch artist image loading for library grid view

Replace per-artist sequential GetArtistMBID + GetArtistImageURL
calls (2 Wails round-trips × N artists) with a single batch
GetArtistImages(names[]) call that:

1. Resolves all names → MBIDs via AllArtistMBIDs() (one DB query)
2. Checks disk cache for each MBID via GetCachedImage (no network)
3. Returns map[name]→dataURL in one Wails bridge round-trip

Only returns already-cached images from the disk cache populated
by the index build. No network fetches triggered — artists whose
images haven't been cached yet keep the initial letter fallback
until the index build resolves them in the background.

Result: all cached artist images appear simultaneously on first
render instead of loading one-by-one over several seconds.

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

@@ -7,16 +7,22 @@ export function BrowseReleaseGroups(arg1:string):Promise<Array<explore.MBRelease
 
 export function BrowseReleases(arg1:string):Promise<Array<explore.MBRelease>>;
 
+export function CheckLibraryMBIDs(arg1:Array<string>):Promise<Record<string, string>>;
+
 export function CoverArtGroupURL(arg1:string):Promise<string>;
 
 export function CoverArtURL(arg1:string):Promise<string>;
 
 export function GetArtistImageURL(arg1:string):Promise<string>;
 
+export function GetArtistMBID(arg1:string):Promise<string>;
+
 export function GetThumbnail(arg1:string,arg2:string,arg3:string):Promise<string>;
 
 export function GetThumbnails(arg1:Array<explore.ThumbnailRequest>):Promise<Record<string, string>>;
 
+export function InvalidateIndexDiscographies():Promise<void>;
+
 export function LookupArtist(arg1:string):Promise<explore.MBArtist>;
 
 export function LookupReleaseGroup(arg1:string):Promise<explore.MBReleaseGroup>;
@@ -33,8 +39,10 @@ export function SetContext(arg1:context.Context):Promise<void>;
 
 export function SimilarArtists(arg1:string):Promise<Array<explore.LBSimilarArtist>>;
 
+export function StartIndexBuild():Promise<void>;
+
+export function StopIndexBuild():Promise<void>;
+
 export function TopRecordingsForArtist(arg1:string):Promise<Array<explore.LBTopRecording>>;
 
-export function CheckLibraryMBIDs(arg1:string[]):Promise<Record<string, string>>;
-
-export function GetArtistMBID(arg1:string):Promise<string>;
+export function GetArtistImages(arg1:string[]):Promise<Record<string, string>>;