Server-side savegames: Rust service with a shared savegame list in the menu, fallback to files

- server/: small axum service storing savegames as files (1 MB each, 500 MB total, rate limit)
- docker compose: second container with a volume; nginx forwards /api/
- JS plugin: asynchronous HTTP requests polled by the game
- Save/Load open a list page (name field, new, overwrite, load, delete with confirmation)
- Without a reachable service the game keeps using savegame files and the browser autosave

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

DESIGN.md

@@ -37,6 +37,7 @@ An original dungeon management game (genre: dungeon building & defence) that run
 ## Menus, savegames, settings
 - **Title screen** (Continue, New Game, Load, Settings, Help) and **pause menu** on Esc (Resume, Save, Load, Settings, Help, Main Menu)
 - Savegames as **JSON** (`*.dcsave`, format tag + version, ~300 KB): **save = download**, **load = file picker or drag & drop** onto the game window
+- **Server savegames** (savegame service in `server/`, shared list without accounts): save under a name / overwrite / load / delete in a list page; the game checks `api/health` at start and **falls back to savegame files** when the service cannot be reached (e.g. local HTML)
 - **Autosave** in the browser (localStorage) every 2 minutes and when the tab is hidden/closed; "Continue" on the title screen
 - Settings: language + sound, camera (rotation/zoom speed, edge scrolling, invert mouse), graphics (tile outlines, wall detail, light sources, interface size), game (speed 1×/2×, hints), **key bindings** (conflicts swap keys; Esc and arrow keys are fixed)
 - Settings are stored in the browser **and** in every savegame; loading a savegame **always applies** its settings