feat(frontend): add (app) route group and rework auth routing

Move authenticated pages under an (app) group with a Sidebar shell, add /auth/login and page-based /login, drop the old explore page and login +server endpoint, and add the server upload helper.

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

@@ -0,0 +1,50 @@
+import type { PageServerLoad, Actions } from './$types';
+import { error, fail, redirect } from '@sveltejs/kit';
+import { createApiClient } from '$lib/server/api';
+
+const MAX_NAME = 100;
+
+export const load: PageServerLoad = async ({ locals, fetch }) => {
+	if (!locals.user) error(401, 'Inicia sesión.');
+
+	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
+	const { data, error: err } = await api.GET('/playlists/users/{userId}', {
+		params: { path: { userId: locals.user.sub }, query: { pageNumber: 1, pageSize: 50 } }
+	});
+
+	if (err || !data) error(502, 'No se pudieron cargar tus playlists.');
+
+	const covers = await Promise.all(
+		data.items.map(async (playlist) => {
+			const { data: tracks } = await api.GET('/playlists/{playlistId}/tracks', {
+				params: { path: { playlistId: playlist.id }, query: { pageNumber: 1, pageSize: 4 } }
+			});
+			return [playlist.id, tracks?.items.map((t) => t.id) ?? []] as const;
+		})
+	);
+
+	return { playlists: data, trackIds: Object.fromEntries(covers) as Record<string, string[]> };
+};
+
+export const actions: Actions = {
+	create: async ({ request, locals, fetch }) => {
+		if (!locals.accessToken) return fail(401, { message: 'Inicia sesión.' });
+
+		const form = await request.formData();
+		const name = String(form.get('name') ?? '').trim();
+		const description = String(form.get('description') ?? '').trim();
+
+		if (name === '' || name.length > MAX_NAME) {
+			return fail(400, { message: 'El nombre es obligatorio (máx. 100 caracteres).', name, description });
+		}
+
+		const api = createApiClient({ fetch, accessToken: locals.accessToken });
+		const { data, error: err } = await api.POST('/playlists', {
+			body: { name, description, pictureIntentId: null }
+		});
+
+		if (err || !data) return fail(502, { message: 'No se pudo crear la playlist.', name, description });
+
+		redirect(303, `/playlists/${data.id}`);
+	}
+};