Title screen, pause menu, settings dialog and savegames

- title screen (continue/new/load/settings/help) and Esc pause menu
- settings: language & sound, camera, graphics, game, rebindable keys;
  stored in localStorage and in every savegame (always applied on load)
- savegames as JSON (.dcsave): download, load via file picker or drag & drop
- autosave to localStorage every 2 minutes and when the tab is hidden/closed
- configurable wall detail, light count, interface size, game speed, hints
- audio behind a cargo feature so native tests run (cargo test --no-default-features)
- savegame round-trip test

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

DESIGN.md

@@ -29,9 +29,14 @@ An original dungeon management game (genre: dungeon building & defence) that run
 - Spells: 0 lightning, X heal, V summon imp
 - Torches: placed automatically when ground is claimed; foreign areas are dimly visible
 
+## 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
+- **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
+
 ## Planned
-- Menu: save (download `*.dcsave`), load (file picker + drag & drop), autosave in the browser (localStorage)
-- Settings: language + sound, camera, graphics, game, key bindings – stored in the browser **and** in the savegame
 - Music: own **XM/MOD replayer in Rust**, shared mixer for music + sound effects (streamed via a JS audio plugin, replaces macroquad audio)
 - Soundtrack (composed as `.xm`, editable in MilkyTracker): title/menu, several calm build tracks, battle, victory/defeat jingles; **the title melody is a recognisable leitmotif in every piece**