User accounts, login before the title screen, own savegames per user, standalone download

- Service: accounts with Argon2 password hashes and session tokens; the first login creates
  the admin (shared savegames of before move to the admin); the admin creates and deletes
  users; everybody can change the password; savegames are stored and listed per user
- Game: login page before the title (only when the service is reachable), with licence and
  repository link; account page (password, logout; users for the admin); text fields with
  Tab/Enter, masked passwords, browser dialog on touch screens; long status messages wrap
- Download Standalone: DungeonCraft.html with the game embedded (built in the Docker image,
  served to logged-in users via nginx auth_request); works from file:// without server and
  opens downloaded savegames
- Without the service (standalone, local files) there is no login, savegame files as before

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

nginx.conf

@@ -12,6 +12,11 @@ server {
         default_type application/manifest+json;
         try_files $uri =404;
     }
+    # the standalone file only for logged-in users (checked by the service)
+    location /standalone/ {
+        auth_request /api/auth;
+        try_files $uri =404;
+    }
     # savegame service (server/). Resolved at request time, so the game still starts without it;
     # the game then falls back to savegame files.
     location /api/ {