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>

tests/core/site.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { API_ENDPOINT, dataPath, moveOldStoredKeys } from '../../src/core/site';
+import { API_ENDPOINT, dataPath, moveOldStoredKeys, sitePath } from '../../src/core/site';
 
 function memoryStore(initial: Record<string, string>) {
   const items = new Map(Object.entries(initial));
@@ -21,6 +21,14 @@ describe('site', () => {
     expect(dataPath('textures')).toBe('/feuerwerk/data/textures');
   });
 
+  it('takes the path from the folder of the open page', () => {
+    expect(sitePath('/feuerwerk/')).toBe('/feuerwerk/');
+    expect(sitePath('/feuerwerk/index.html')).toBe('/feuerwerk/');
+    expect(sitePath('/feuerwerk/play.html')).toBe('/feuerwerk/');
+    expect(sitePath('/')).toBe('/');
+    expect(sitePath('/play.html')).toBe('/');
+  });
+
   it('moves the keys stored under the old name and leaves the others alone', () => {
     const store = memoryStore({
       'game-maker.panel-sizes': '{"left":300}',