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>

tests/editor/panel-sizes.test.ts

@@ -2,7 +2,7 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { keepPanelSize, panelSize, resetPanelSizes } from '../../src/editor/panelSizes';
 
-const STORAGE_KEY = 'game-maker.panel-sizes';
+const STORAGE_KEY = 'feuerwerk.panel-sizes';
 
 function memoryStore(initial: Record<string, string> = {}) {
   const items = new Map(Object.entries(initial));

tests/editor/sections.test.ts

@@ -264,7 +264,7 @@ describe('the memory of folded groups', () => {
     setFolded(key, true);
     expect(isFolded(key)).toBe(true);
     resetFolds();
-    vi.stubGlobal('window', { localStorage: memoryStore({ 'game-maker.folded-sections': '{broken' }) });
+    vi.stubGlobal('window', { localStorage: memoryStore({ 'feuerwerk.folded-sections': '{broken' }) });
     expect(isFolded(key)).toBe(false);
   });
 

tests/editor/view-tween.test.ts

@@ -405,7 +405,7 @@ describe('the general settings', () => {
     keepGeneralSettings({ zoomTransitionMs: 350 });
     resetGeneralSettings();
     expect(generalSettings().zoomTransitionMs).toBe(350);
-    expect(JSON.parse(store.items.get('game-maker.general-settings') ?? '')).toEqual({
+    expect(JSON.parse(store.items.get('feuerwerk.general-settings') ?? '')).toEqual({
       zoomTransitionMs: 350,
       showEmptyAssetGroups: false,
       customCursor: true,
@@ -437,7 +437,7 @@ describe('the general settings', () => {
     resetGeneralSettings();
     expect(generalSettings().customCursor).toBe(false);
     // Settings kept before the switch existed show it.
-    store.setItem('game-maker.general-settings', JSON.stringify({ zoomTransitionMs: 100 }));
+    store.setItem('feuerwerk.general-settings', JSON.stringify({ zoomTransitionMs: 100 }));
     resetGeneralSettings();
     expect(generalSettings().customCursor).toBe(true);
   });