refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/src/lib/theme/theme.css

@@ -0,0 +1,22 @@
+:root {
+	--mf-bg: rgb(9, 9, 9);
+	--mf-elevated: #0b0b0d;
+
+	--mf-text: #c7c6c5;
+	--mf-text-2: #a5a4a3;
+	--mf-text-3: #8c8b8a;
+	--mf-text-4: #7f7e7d;
+
+	--mf-surface: rgb(18, 18, 18);
+	--mf-surface-2: rgba(255, 255, 255, 0.06);
+	--mf-surface-hover: rgba(255, 255, 255, 0.09);
+
+	--mf-border: rgba(255, 255, 255, 0.08);
+	--mf-border-strong: rgba(255, 255, 255, 0.12);
+	--mf-track: rgba(255, 255, 255, 0.12);
+
+	--mf-accent: oklch(60% 0.17 145);
+	--mf-accent-60: color-mix(in oklch, var(--mf-accent) 80%, transparent);
+	--mf-radius: 12px;
+	--mf-sidebar-w: 264px;
+}