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>

Cargo.toml

@@ -4,7 +4,14 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
-macroquad = { version = "0.4", features = ["audio"] }
+macroquad = { version = "0.4" }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+[features]
+default = ["audio"]
+# macroquad audio (needs libasound natively); disable for native tests: cargo test --no-default-features
+audio = ["macroquad/audio"]
 
 [profile.release]
 opt-level = 3