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/map.rs

@@ -1,5 +1,6 @@
 // Map, tiles, generation and path finding.
 
+use serde::{Deserialize, Serialize};
 use std::cmp::Reverse;
 use std::collections::BinaryHeap;
 
@@ -7,7 +8,7 @@ pub const MW: usize = 64;
 pub const MH: usize = 64;
 pub const N: usize = MW * MH;
 
-#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
 pub enum Kind {
     Rock,
     Earth,
@@ -23,24 +24,34 @@ pub enum Kind {
     Gate,
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
 pub enum Owner {
     None,
     Player,
     Hero,
 }
 
-#[derive(Clone, Copy)]
+// Short serde names keep savegames small (the map has 4096 tiles)
+#[derive(Clone, Copy, Serialize, Deserialize)]
 pub struct Tile {
+    #[serde(rename = "k")]
     pub kind: Kind,
+    #[serde(rename = "o")]
     pub owner: Owner,
-    pub hp: f32,      // dig resistance
-    pub gold: i32,    // gold left (gold vein)
+    #[serde(rename = "h")]
+    pub hp: f32, // dig resistance
+    #[serde(rename = "g")]
+    pub gold: i32, // gold left (gold vein)
+    #[serde(rename = "m")]
     pub marked: bool, // marked for digging
+    #[serde(rename = "r")]
     pub reserved: u32, // entity id + 1 working on this tile
-    pub food: f32,    // mushrooms in the mushroom garden
-    pub bed: u32,     // entity id + 1 sleeping here
-    pub var: u8,      // texture variant
+    #[serde(rename = "f")]
+    pub food: f32, // mushrooms in the mushroom garden
+    #[serde(rename = "b")]
+    pub bed: u32, // entity id + 1 sleeping here
+    #[serde(rename = "v")]
+    pub var: u8, // texture variant
 }
 
 impl Tile {
@@ -74,6 +85,7 @@ pub fn inb(x: i32, y: i32) -> bool {
 pub const DIR4: [(i32, i32); 4] = [(0, -1), (1, 0), (0, 1), (-1, 0)];
 pub const DIR8: [(i32, i32); 8] = [(0, -1), (1, 0), (0, 1), (-1, 0), (1, -1), (1, 1), (-1, 1), (-1, -1)];
 
+#[derive(Serialize, Deserialize)]
 pub struct Rng(pub u32);
 impl Rng {
     pub fn next(&mut self) -> u32 {
@@ -95,6 +107,7 @@ impl Rng {
     }
 }
 
+#[derive(Serialize, Deserialize)]
 pub struct Map {
     pub t: Vec<Tile>,
     pub core: (i32, i32),  // top-left corner of the core (3x3)