feat: local-first data pipeline — check library before API calls

Audit of all external calls across explore components, with local
sources checked first:

1. loadThumbnails: seeds thumbnailCache from library album cover
   art (CoverArtMedium/Small by MBID) before building the API
   request list. Library albums show cover art instantly; only
   non-library albums hit the GetThumbnails API.

2. loadArtistImages: seeds artistImageCache from library store
   (ImageMedium/Small by MBID) before the sequential API loop.
   Library artists show images instantly; only non-library artists
   hit GetArtistImageURL.

3. checkLibrary (explore-view): checks library store MBIDs
   frontend-side for artists and albums. Only falls back to
   CheckLibraryMBIDs API for recordings (not in library store).

4. checkLibrary (artist-details): same frontend-first approach
   using library album MBIDs.

5. hydrateFromCache (artist-details): now also checks library
   store directly for artist images when explore cache is empty
   (handles direct navigation without prior search).

frontend/src/components/explore-artist-details/explore-artist-details.ts

@@ -773,15 +773,26 @@ export class ExploreArtistDetails extends LitElement {
      * Shows cached data instantly before API calls complete.
      */
     private hydrateFromCache(mbid: string) {
-        // Artist image from explore cache (populated by search results).
+        // Artist image: check explore cache first, then library store.
-        const cachedArtist = exploreCache.getArtist(mbid);
+        if (!this.artistImageURL) {
-        if (cachedArtist) {
+            const cachedArtist = exploreCache.getArtist(mbid);
-            if (cachedArtist.imageURL) {
+            if (cachedArtist) {
-                this.artistImageURL = cachedArtist.imageURL;
+                this.artistImageURL = cachedArtist.imageURL
-            } else if (cachedArtist.imageMedium) {
+                    || cachedArtist.imageMedium
-                this.artistImageURL = cachedArtist.imageMedium;
+                    || cachedArtist.imageSmall
-            } else if (cachedArtist.imageSmall) {
+                    || '';
-                this.artistImageURL = cachedArtist.imageSmall;
+            }
+        }
+
+        if (!this.artistImageURL) {
+            const cachedArtists = libraryStore.cachedArtists;
+            if (cachedArtists) {
+                for (const a of cachedArtists) {
+                    if (a.MBID === mbid) {
+                        this.artistImageURL = a.ImageMedium || a.ImageSmall || '';
+                        break;
+                    }
+                }
             }
         }
 
@@ -930,6 +941,30 @@ export class ExploreArtistDetails extends LitElement {
     }
 
     private async checkLibrary() {
+        // Check frontend-side first.
+        const cachedAlbums = libraryStore.cachedAlbums;
+        if (cachedAlbums) {
+            const localMBIDs = new Set<string>();
+            for (const a of cachedAlbums) {
+                if (a.MBID) localMBIDs.add(a.MBID);
+            }
+
+            let updated = false;
+
+            for (const rg of this.releaseGroups) {
+                if (rg.mbid && localMBIDs.has(rg.mbid)) {
+                    this.libraryMBIDs.add(rg.mbid);
+                    updated = true;
+                }
+            }
+
+            if (updated) {
+                this.requestUpdate();
+                return;
+            }
+        }
+
+        // Fallback to backend.
         const mbids: string[] = [];
 
         for (const rg of this.releaseGroups) {

frontend/src/components/explore-view/explore-view.ts

@@ -858,7 +858,27 @@ export class ExploreView extends LitElement {
             return;
         }
 
-        // Collect MBIDs that need fetching.
+        // Seed thumbnails from library album cover art (instant, no API).
+        const cachedAlbums = libraryStore.cachedAlbums;
+        if (cachedAlbums) {
+            const libAlbumsByMBID = new Map<string, string>();
+            for (const a of cachedAlbums) {
+                if (a.MBID && (a.CoverArtMedium || a.CoverArtSmall)) {
+                    libAlbumsByMBID.set(a.MBID, a.CoverArtMedium || a.CoverArtSmall);
+                }
+            }
+
+            for (const rg of this.results.releaseGroups) {
+                if (!this.thumbnailCache.has(rg.mbid)) {
+                    const localArt = libAlbumsByMBID.get(rg.mbid) || (rg as any)._coverArt;
+                    if (localArt) {
+                        this.thumbnailCache.set(rg.mbid, localArt);
+                    }
+                }
+            }
+        }
+
+        // Collect MBIDs that still need fetching from the API.
         const requests: ThumbnailRequest[] = [];
 
         for (const rg of this.results.releaseGroups) {
@@ -917,7 +937,29 @@ export class ExploreView extends LitElement {
     private async loadArtistImages() {
         if (!this.results?.artists?.length) return;
 
-        // Load sequentially to avoid hammering the MB rate limiter.
+        // Seed from library store first (instant, no API).
+        const cachedArtists = libraryStore.cachedArtists;
+        if (cachedArtists) {
+            const libByMBID = new Map<string, string>();
+            for (const a of cachedArtists) {
+                if (a.MBID && (a.ImageMedium || a.ImageSmall)) {
+                    libByMBID.set(a.MBID, a.ImageMedium || a.ImageSmall);
+                }
+            }
+
+            for (const a of this.results.artists) {
+                if (!this.artistImageCache.has(a.mbid) && a.mbid) {
+                    const local = libByMBID.get(a.mbid) || (a as any)._imageMedium || (a as any)._imageSmall;
+                    if (local) {
+                        this.artistImageCache.set(a.mbid, local);
+                    }
+                }
+            }
+
+            this.requestUpdate();
+        }
+
+        // Fetch remaining from API (only artists not yet resolved).
         for (const a of this.results.artists) {
             if (this.artistImageCache.has(a.mbid)) continue;
 
@@ -942,14 +984,50 @@ export class ExploreView extends LitElement {
     private async checkLibrary() {
         if (!this.results) return;
 
-        const mbids: string[] = [];
+        // Check frontend-side first using library store MBIDs.
+        const cachedArtists = libraryStore.cachedArtists;
+        const cachedAlbums = libraryStore.cachedAlbums;
+        const localMBIDs = new Set<string>();
+
+        if (cachedArtists) {
+            for (const a of cachedArtists) {
+                if (a.MBID) localMBIDs.add(a.MBID);
+            }
+        }
+
+        if (cachedAlbums) {
+            for (const a of cachedAlbums) {
+                if (a.MBID) localMBIDs.add(a.MBID);
+            }
+        }
+
+        let updated = false;
 
         for (const a of this.results.artists ?? []) {
-            if (a.mbid) mbids.push(a.mbid);
+            if (a.mbid && localMBIDs.has(a.mbid)) {
+                this.libraryMBIDs.add(a.mbid);
+                updated = true;
+            }
         }
 
         for (const rg of this.results.releaseGroups ?? []) {
-            if (rg.mbid) mbids.push(rg.mbid);
+            if (rg.mbid && localMBIDs.has(rg.mbid)) {
+                this.libraryMBIDs.add(rg.mbid);
+                updated = true;
+            }
+        }
+
+        if (updated) {
+            this.requestUpdate();
+            return;
+        }
+
+        // Fallback to backend check for recordings and edge cases
+        // (recordings aren't in the library store cache).
+        const mbids: string[] = [];
+
+        for (const r of this.results.recordings ?? []) {
+            if (r.mbid) mbids.push(r.mbid);
         }
 
         if (mbids.length === 0) return;