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>

README.md

@@ -1,4 +1,4 @@
-# Game Maker
+# Feuerwerk
 
 [![game-maker](https://git.nockal.com/meleki/game-maker/actions/workflows/ci.yml/badge.svg?branch=main)](https://git.nockal.com/meleki/game-maker/actions?workflow=ci.yml)
 
@@ -8,7 +8,7 @@ all in one program, all drawn inside a single WebGL canvas.
 
 ![](screenshot.png)
 
-Live: <https://nockal.com/game-maker/> · Widget playground: <https://nockal.com/game-maker/?playground>
+Live: <https://nockal.com/feuerwerk/> · Widget playground: <https://nockal.com/feuerwerk/?playground>
 
 **The documentation is the wiki:** <https://git.nockal.com/meleki/game-maker/wiki> – every concept,
 node, tab and shortcut, plus the architecture and the development workflow. This file is the short
@@ -39,7 +39,7 @@ Node 22 is required.
 
 ```bash
 npm install
-npm run dev          # http://localhost:5175/game-maker/dist/ – API calls are proxied to production
+npm run dev          # http://localhost:5175/feuerwerk/dist/ – API calls are proxied to production
 npm run check        # typecheck + lint + format:check + unit tests – the gate CI runs too
 npm run build        # writes dist/ and regenerates index.html; the folder is what nginx serves
 ```