refactor: tighten the home typography and card craft

The playlist cards were showing "No description was pr…" because the API
stored that English sentence whenever a playlist was created without a
description. Descriptions are optional now and the old rows are cleared.

Section headings drop to 18/20px so they stop competing with the content,
and the space moved from inside the sections to between them.

The three media cards repeated the same anchor, art class and label
block, so they share a MediaCard that also carries the hover lift the
track tiles already had.

backend/Musify.Infrastructure/Persistence/Migrations/DatabaseModelSnapshot.cs

@@ -409,7 +409,6 @@ namespace Musify.Infrastructure.Persistence.Migrations
                         .HasColumnType("timestamp with time zone");
 
                     b.Property<string>("Description")
-                        .IsRequired()
                         .HasMaxLength(256)
                         .HasColumnType("character varying(256)");