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>

src/sound.rs

@@ -3,7 +3,25 @@
 // Events without a registered sound stay silent.
 
 use crate::game::{Sfx, SfxEvent, SFX_ALL};
+#[cfg(feature = "audio")]
 use macroquad::audio::{load_sound_from_bytes, play_sound, PlaySoundParams, Sound};
+#[cfg(not(feature = "audio"))]
+use stub::*;
+
+// Silent stand-in when the "audio" feature is off (native tests)
+#[cfg(not(feature = "audio"))]
+mod stub {
+    #[derive(Clone)]
+    pub struct Sound;
+    pub struct PlaySoundParams {
+        pub looped: bool,
+        pub volume: f32,
+    }
+    pub async fn load_sound_from_bytes(_: &[u8]) -> Result<Sound, ()> {
+        Err(())
+    }
+    pub fn play_sound(_: &Sound, _: PlaySoundParams) {}
+}
 use macroquad::prelude::*;
 
 pub struct SoundBank {
@@ -33,10 +51,6 @@ impl SoundBank {
     // procedurally generated sounds will be registered here
     pub async fn generate_all(&mut self) {}
 
-    pub fn change_volume(&mut self, d: f32) {
-        self.volume = (self.volume + d).clamp(0.0, 1.0);
-    }
-
     // minimum gap between two identical sounds (avoids a wall of noise)
     fn min_gap(sfx: Sfx) -> f64 {
         match sfx {