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>

README.md

@@ -34,12 +34,17 @@ python3 -m http.server 8080 -d www   # open http://localhost:8080
 
 ### Testing the game logic
 
-The game logic (`src/game.rs`, `src/map.rs`) does not depend on macroquad and contains a headless simulation test.
+The game logic (`src/game.rs`, `src/map.rs`) does not depend on macroquad. The tests (a headless simulation of whole games and a savegame round trip) run natively without the audio feature:
+
+```sh
+cargo test --release --no-default-features
+```
 
 ## Controls
 
 | Input | Action |
 |---|---|
+| Esc | Pause menu (save, load, settings) |
 | Left click / drag | Use the active tool (dig and rooms: drag a rectangle) |
 | Right click | Drop a unit from your hand |
 | Right drag, WASD, arrows, screen edge | Move the camera |
@@ -51,6 +56,12 @@ The game logic (`src/game.rs`, `src/map.rs`) does not depend on macroquad and co
 | G | Tile outlines on/off |
 | H | Help |
 
+All keys except Esc and the arrow keys can be changed in Settings → Keys.
+
+## Savegames
+
+Use *Save Game* in the pause menu to download a `.dcsave` file (JSON). Load it again via *Load Game* or by dropping the file onto the game window. The game also autosaves into the browser every two minutes and when the tab is closed; *Continue* on the title screen resumes from there.
+
 ## Adding a language
 
 1. Copy `lang/en.lang` to `lang/<code>.lang` and translate the texts (placeholders `{0}`, `{1}` stay as they are).