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/explore/Service.d.ts

@@ -1,7 +1,6 @@
 // Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
 // This file is automatically generated. DO NOT EDIT
 import {explore} from '../models';
-import {http} from '../models';
 import {context} from '../models';
 
 export function BrowseReleaseGroups(arg1:string):Promise<Array<explore.MBReleaseGroup>>;
@@ -38,8 +37,6 @@ export function SearchArtists(arg1:string):Promise<Array<explore.MBArtist>>;
 
 export function SearchLocal(arg1:string):Promise<explore.MBSearchResult>;
 
-export function SearchLocalHandler():Promise<http.Handler>;
-
 export function SearchRecordings(arg1:string):Promise<Array<explore.MBRecording>>;
 
 export function SearchReleaseGroups(arg1:string):Promise<Array<explore.MBReleaseGroup>>;

frontend/wailsjs/go/explore/Service.js

@@ -70,10 +70,6 @@ export function SearchLocal(arg1) {
   return window['go']['explore']['Service']['SearchLocal'](arg1);
 }
 
-export function SearchLocalHandler() {
-  return window['go']['explore']['Service']['SearchLocalHandler']();
-}
-
 export function SearchRecordings(arg1) {
   return window['go']['explore']['Service']['SearchRecordings'](arg1);
 }