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/app.d.ts

@@ -0,0 +1,12 @@
+import type { SessionUser } from '$lib/types';
+
+declare global {
+	namespace App {
+		interface Locals {
+			user: SessionUser | null;
+			accessToken: string | null;
+		}
+	}
+}
+
+export {};