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>

server/Cargo.toml

@@ -10,6 +10,8 @@ axum = "0.8"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "fs", "sync", "time"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+argon2 = "0.5"
+getrandom = "0.2"
 
 [profile.release]
 opt-level = 3