Serve the editor from feuerwerk.nockal.com as well

The subdomain proxies to the same folder, but the build had the path
/feuerwerk/ baked in. On feuerwerk.nockal.com the page loaded its
assets, the API and the data URLs from /feuerwerk/... and got 404. The
session cookie had Path=/feuerwerk/, so the browser would never have
sent it to /api/ there either.

- site.ts takes the base path from the folder of the open page:
  /feuerwerk/ on nockal.com, / on the subdomain. Release links point at
  the address you are working on.
- vite.config.ts builds with a relative base. The copy plugin rewrites
  the pages to ./dist/assets/ and stops the build if it finds nothing
  to rewrite. The dev server now runs at http://localhost:5175/.
- Auth.php sets the cookie without a Path, so it belongs to the API
  folder the browser sees. Sign-in and sign-out also clear the old
  /feuerwerk/ cookie, otherwise it would keep signing in after a
  logout. Existing sessions stay valid.
- README: the new dev server address.

Tests:
- curl on both addresses: pages, JS, CSS and API 200
- sign-in, session, sign-out on both addresses
- e2e 70 against both addresses: 13/13 each, no CSP violation
- tsc, lint, prettier clean, site and releases tests 14/14
- PHP tests 7 of 8 files green, videos.test.php fails only because
  the web-php-fpm container has no ffmpeg

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/site.ts

@@ -1,5 +1,15 @@
-/** The path the site is served under: the editor, the play page, the API and every data file. */
-export const SITE_PATH = '/feuerwerk/';
+/**
+ * The path the site is served under: the editor, the play page, the API and every data file. It
+ * is the folder of the open page, so one build answers at https://nockal.com/feuerwerk/ and at
+ * https://feuerwerk.nockal.com/ alike. Away from a page (Node, the tests) it is the nginx path.
+ */
+export const SITE_PATH = sitePath(typeof location === 'undefined' ? undefined : location.pathname);
+
+/** The folder of a page path: `/feuerwerk/play.html` and `/feuerwerk/` give `/feuerwerk/`. */
+export function sitePath(pagePath: string | undefined): string {
+  if (pagePath === undefined) return '/feuerwerk/';
+  return pagePath.slice(0, pagePath.lastIndexOf('/') + 1);
+}
 
 /** The API's one entry point. */
 export const API_ENDPOINT = `${SITE_PATH}api/index.php`;