Rename Game Maker to Feuerwerk and serve it at /feuerwerk/ (#422)

The editor gets a name of its own that fits beside Brandmauer,
Glutspeicher and Feuerwehr. This is stage 1, 2 and 4 of #422: the
name, the URL and the stored state in the browser. Repository, CI
runners, containers and the GAME_MAKER_* variables follow in stage 3.

- All paths come from SITE_PATH in src/core/site.ts: the API endpoint,
  the data folders, the play page and the base in vite.config.ts. A
  later move is one line instead of fifteen.
- The window title, the sign-in screen, the playground and the README
  say Feuerwerk. The package, __feuerwerkPlay, the take worklet and the
  writing app in Ogg and Matroska follow.
- The session cookie is now feuerwerk_session, so everyone signs in
  once more.
- localStorage keys are now feuerwerk.*. moveOldStoredKeys copies the
  old game-maker.* keys over on the first start, so settings, panel
  sizes and save games survive.
- tools/ci/serve.sh serves the build under /feuerwerk/. The variable
  GAME_MAKER_BASE_URL keeps its name until stage 3.

Outside the repository, nockal.com/feuerwerk/ is a second symlink to
the same tree. nginx sends the bare /game-maker/ folder there with a
301. Everything else under /game-maker/ is still served directly, so
shared play links and the API keep working.

Finding: when nginx redirects a path without a trailing slash or with
a query, the Brandmauer answers with its own 307 to the same URL plus
a slash, placed after the query. A play link would lose its token that
way, which is why only the bare folder is redirected.

Tests:
- Gate on melektaus-atx: CHECK EXIT 0, 307 files, 4756 tests
  (4 new in site.test.ts).
- Counter-check: when the migration misses the old prefix, 3 of the
  4 new tests fail.
- e2e 60 under /feuerwerk/: ALL PASSED, 20 checks.
- From outside: /feuerwerk/ 200, /game-maker/ 301, an old play
  link 200.

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

src/core/site.ts

@@ -0,0 +1,39 @@
+/** The path the site is served under: the editor, the play page, the API and every data file. */
+export const SITE_PATH = '/feuerwerk/';
+
+/** The API's one entry point. */
+export const API_ENDPOINT = `${SITE_PATH}api/index.php`;
+
+/** Where the uploaded files of one kind are served, e.g. `dataPath('textures')`. */
+export function dataPath(kind: string): string {
+  return `${SITE_PATH}data/${kind}`;
+}
+
+/** The prefix of every `localStorage` key while the editor was called Game Maker. */
+const OLD_KEY_PREFIX = 'game-maker.';
+const KEY_PREFIX = 'feuerwerk.';
+
+type KeyStore = Pick<Storage, 'length' | 'key' | 'getItem' | 'setItem' | 'removeItem'>;
+
+/**
+ * Moves what a browser kept under the old name to the new one: settings, panel sizes, folds and
+ * saves survive the rename. A key the new name already has keeps its value. Answers how many moved.
+ */
+export function moveOldStoredKeys(storage: KeyStore): number {
+  const old: string[] = [];
+  for (let i = 0; i < storage.length; i++) {
+    const key = storage.key(i);
+    if (key?.startsWith(OLD_KEY_PREFIX)) old.push(key);
+  }
+  let moved = 0;
+  for (const key of old) {
+    const value = storage.getItem(key);
+    const renamed = KEY_PREFIX + key.slice(OLD_KEY_PREFIX.length);
+    if (value !== null && storage.getItem(renamed) === null) {
+      storage.setItem(renamed, value);
+      moved++;
+    }
+    storage.removeItem(key);
+  }
+  return moved;
+}