feat(playlist): use first album cover art for playlists

Add logic to find and use album cover art as a fallback when playlist
cover art is missing, both for local and online playlists. Update
database schema and upsert logic to store album picture URLs for
online tracks. Improve UI to display fallback cover art when needed.

src/routes/playlists/[name]/+page.svelte

@@ -3,7 +3,7 @@
   import { page } from '$app/stores';
   import { settings, loadSettings } from '$lib/stores/settings';
   import { scanPlaylists } from '$lib/library/scanner';
-  import { loadPlaylistTracks, findPlaylistArt } from '$lib/library/playlist';
+  import { loadPlaylistTracks, findPlaylistArt, findPlaylistCoverFallback } from '$lib/library/playlist';
   import CollectionView from '$lib/components/CollectionView.svelte';
   import type { Track, PlaylistWithTracks } from '$lib/types/track';
 
@@ -58,6 +58,11 @@
       playlistData = tracksData;
       coverArtPath = coverPath;
 
+      // If no cover art found, try fallback from first track's album
+      if (!coverArtPath && playlistData.tracks.length > 0) {
+        coverArtPath = await findPlaylistCoverFallback(playlistData.tracks, $settings.musicFolder);
+      }
+
       loading = false;
     } catch (e) {
       error = 'Error loading playlist: ' + (e as Error).message;

src/routes/services/deezer/playlists/[id]/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import { page } from '$app/stores';
+  import { convertFileSrc } from '@tauri-apps/api/core';
   import { deezerAuth } from '$lib/stores/deezer';
   import { deezerAPI } from '$lib/services/deezer';
   import { settings } from '$lib/stores/settings';
@@ -14,6 +15,7 @@
   import { TrackExistenceCache } from '$lib/library/trackMatcher';
   import { addDeezerTrackToQueue } from '$lib/services/deezer/addToQueue';
   import { downloadDeezerPlaylist } from '$lib/services/deezer/playlistDownloader';
+  import { findPlaylistCoverFallback } from '$lib/library/playlist';
   import DeezerCollectionView from '$lib/components/DeezerCollectionView.svelte';
   import type { Track } from '$lib/types/track';
   import type { DeezerTrack } from '$lib/types/deezer';
@@ -85,6 +87,47 @@
 
       // Check track existence after loading tracks
       await checkTrackExistence();
+
+      // If no valid cover art from Deezer playlist, fetch and use first track's album cover
+      if ((!playlistPicture || !playlistPicture.startsWith('http')) && tracks.length > 0) {
+        const cachedTracks = isFavoriteTracks
+          ? await getCachedTracks()
+          : await getCachedPlaylistTracks(playlistId);
+
+        if (cachedTracks.length > 0) {
+          // If we have cached album picture, use it
+          if (cachedTracks[0].album_picture) {
+            playlistPicture = cachedTracks[0].album_picture;
+          } else if ($deezerAuth.arl && cachedTracks[0].track_id) {
+            // Fetch album data from API to get cover
+            try {
+              deezerAPI.setArl($deezerAuth.arl);
+              const trackData = await deezerAPI.getTrackData(cachedTracks[0].track_id);
+              if (trackData && trackData.ALB_PICTURE) {
+                const albumCoverUrl = `https://e-cdns-images.dzcdn.net/images/cover/${trackData.ALB_PICTURE}/500x500-000000-80-0-0.jpg`;
+                playlistPicture = albumCoverUrl;
+
+                // Update cache with the album picture
+                const database = await import('$lib/library/deezer-database').then(m => m.initDeezerDatabase());
+                await database.execute(
+                  'UPDATE deezer_playlist_tracks SET album_picture = $1 WHERE track_id = $2',
+                  [albumCoverUrl, cachedTracks[0].track_id]
+                );
+              }
+            } catch (err) {
+              console.error('Failed to fetch album cover:', err);
+            }
+          }
+        }
+
+        // Final fallback to local files
+        if ((!playlistPicture || !playlistPicture.startsWith('http')) && $settings.musicFolder) {
+          const fallbackPath = await findPlaylistCoverFallback(tracks, $settings.musicFolder);
+          if (fallbackPath) {
+            playlistPicture = convertFileSrc(fallbackPath);
+          }
+        }
+      }
     } catch (e) {
       error = 'Error loading playlist: ' + (e instanceof Error ? e.message : String(e));
     } finally {