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/routes/(app)/playlists/+page.server.ts

@@ -0,0 +1,71 @@
+import type { PageServerLoad, Actions } from './$types';
+import { fail, redirect } from '@sveltejs/kit';
+import {
+	createApiClient,
+	playlistCoverTrackIds,
+	requireAccessTokenAction,
+	requireUser,
+	unwrapOrError
+} from '$lib/server/api';
+import { uploadPresignedImage } from '$lib/server/upload';
+
+const MAX_NAME = 100;
+
+export const load: PageServerLoad = async ({ locals, url, fetch }) => {
+	const user = requireUser(locals, url);
+	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
+	const result = await api.GET('/playlists/users/{userId}', {
+		params: { path: { userId: user.sub }, query: { pageNumber: 1, pageSize: 50 } }
+	});
+
+	const data = unwrapOrError(result, 'No se pudieron cargar tus playlists.');
+	const trackIds = await playlistCoverTrackIds(
+		api,
+		data.items.map((p) => p.id)
+	);
+
+	return { playlists: data, trackIds };
+};
+
+export const actions: Actions = {
+	create: async ({ request, locals, fetch }) => {
+		const accessToken = requireAccessTokenAction(locals);
+		if (typeof accessToken !== 'string') return accessToken;
+
+		const form = await request.formData();
+		const name = String(form.get('name') ?? '').trim();
+		const description = String(form.get('description') ?? '').trim();
+		const cover = form.get('cover');
+
+		if (name === '' || name.length > MAX_NAME) {
+			return fail(400, {
+				message: 'El nombre es obligatorio (máx. 100 caracteres).',
+				name,
+				description
+			});
+		}
+
+		const api = createApiClient({ fetch, accessToken });
+
+		let pictureIntentId: string | null = null;
+		if (cover instanceof File && cover.size > 0) {
+			const result = await uploadPresignedImage(
+				(args) => api.POST('/playlists/upload-picture', { body: args }),
+				cover
+			);
+			if ('failMessage' in result) {
+				return fail(502, { message: result.failMessage, name, description, detail: result.detail });
+			}
+			pictureIntentId = result.intentId;
+		}
+
+		const { data, error: err } = await api.POST('/playlists', {
+			body: { name, description, pictureIntentId }
+		});
+
+		if (err || !data)
+			return fail(502, { message: 'No se pudo crear la playlist.', name, description });
+
+		redirect(303, `/playlists/${data.id}`);
+	}
+};