perf(portal): parallelize API calls on mount (#673)

The portal page was making 4 API calls sequentially during mount:
- loadMyTracks()
- loadArtistProfile()
- loadMyAlbums()
- loadMyPlaylists()

Each waited for the previous to complete before starting. Since these
are independent, run them in parallel with Promise.all() to reduce
time-to-interactive.

Expected improvement: ~300-800ms reduction in LCP depending on network
conditions (from sequential to parallel latency).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[e-mail]>

frontend/src/routes/portal/+page.svelte

···
 		}
 
 		try {
-			await loadMyTracks();
-			await loadArtistProfile();
-			await loadMyAlbums();
-			await loadMyPlaylists();
+			await Promise.all([
+				loadMyTracks(),
+				loadArtistProfile(),
+				loadMyAlbums(),
+				loadMyPlaylists()
+			]);
 		} catch (_e) {
 			console.error('error loading portal data:', _e);
 			error = 'failed to load portal data';