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>

lang/en.lang

@@ -198,3 +198,72 @@ tile.marked = marked for digging
 # ---- language selection
 langsel.title = Language
 langsel.hint = Click a language. You can change it later in the settings.
+
+# ---- menus
+menu.subtitle = Dig. Build. Defend. Conquer.
+menu.continue = Continue
+menu.new = New Game
+menu.load = Load Game
+menu.save = Save Game
+menu.settings = Settings
+menu.help = Help
+menu.resume = Resume
+menu.main = Main Menu
+menu.back = Back
+menu.paused = Paused
+menu.drop_hint = Tip: you can also drop a .dcsave file onto the game window to load it.
+
+# ---- settings
+set.tab.sound = Language & Sound
+set.tab.camera = Camera
+set.tab.graphics = Graphics
+set.tab.game = Game
+set.tab.keys = Keys
+set.language = Language
+set.volume = Sound volume
+set.music = Music volume
+set.mute = Mute
+set.rotate_speed = Rotation speed
+set.zoom_speed = Zoom speed
+set.edge_scroll = Screen edge scrolling
+set.invert = Invert mouse rotation
+set.outlines = Tile outlines
+set.wall_detail = Wall detail
+set.lights = Light sources
+set.ui_scale = Interface size
+set.game_speed = Game speed
+set.hints = Hints
+set.keys_hint = Click an action and press a key. Keys already in use are swapped.
+set.press_key = Press a key for: {0}
+set.esc_cancel = Esc cancels
+set.reset_keys = Reset keys
+val.on = On
+val.off = Off
+val.low = Low
+val.medium = Medium
+val.high = High
+val.normal = Normal
+val.large = Large
+
+# ---- key binding names
+key.cam_forward = Camera forward
+key.cam_back = Camera back
+key.cam_left = Camera left
+key.cam_right = Camera right
+key.rotate_left = Rotate left
+key.rotate_right = Rotate right
+key.tilt_up = Tilt up
+key.tilt_down = Tilt down
+key.pause = Pause
+key.mute = Sound on/off
+key.volume_up = Volume up
+key.volume_down = Volume down
+key.outlines = Tile outlines
+key.help = Help
+
+# ---- savegames
+save.saved = Game saved: {0}
+save.loaded = Game loaded.
+save.err_invalid = This file is not a valid DungeonCraft savegame.
+save.err_version = This savegame comes from a newer version of DungeonCraft.
+save.choose = Choose a savegame file ...